Sure, I'll help you with the answers and explanations for each question.

**Question 41:** I have been saving money because I____________ buy a computer
- **A.** shall
- **B.** will
- **C.** am going to
- **D.** would

**Answer:** **C. am going to**
**Explanation:** "Am going to" is used to indicate a plan or intention. Since the speaker has been saving money with the intention of buying a computer, this is the most appropriate choice.

**Question 42:** By 8 o’clock yesterday, the spaceship______20 hours around the earth
- **A.** will travel
- **B.** has been traveling
- **C.** had been traveling
- **D.** will have been traveling

**Answer:** **C. had been traveling**
**Explanation:** The past perfect continuous tense "had been traveling" is used to describe an action that was ongoing up until a certain point in the past.

**Question 43:** Over 20 years______but I never forgot the time we first met each other.
- **A.** went
- **B.** have gone up
- **C.** has gone by
- **D.** has gone out

**Answer:** **C. has gone by**
**Explanation:** "Has gone by" is the correct phrase to indicate the passage of time.

**Question 44:** The child kept the present that she______.
- **A.** has been given
- **B.** had been given
- **C.** had given
- **D.** has given

**Answer:** **B. had been given**
**Explanation:** The past perfect passive "had been given" is used to indicate that the action of giving the present happened before the child kept it.

**Question 45:** That pipe______for ages-we must get it mended.
- **A.** has been leaking
- **B.** has been leaked
- **C.** had been leaked
- **D.** had been leaking

**Answer:** **A. has been leaking**
**Explanation:** The present perfect continuous "has been leaking" is used to describe an action that started in the past and is still continuing.

**Question 46:** I used my calculator; otherwise it______longer
- **A.** will take
- **B.** would take
- **C.** took
- **D.** would have taken

**Answer:** **D. would have taken**
**Explanation:** "Would have taken" is used in the third conditional to describe a hypothetical situation in the past that did not happen.

**Question 47:** How many musical notes of the 11,000 tones that the human ear can distinguish______in the musical scale?
- **A.** is it
- **B.** it is
- **C.** are there
- **D.** there are

**Answer:** **C. are there**
**Explanation:** The correct structure for this question is "How many... are there...?"

**Question 48:** You______to our conversation. It was private.
- **A.** haven’t been listening
- **B.** shouldn’t have been listening
- **C.** couldn’t have been listening
- **D.** hadn’t been listening

**Answer:** **B. shouldn’t have been listening**
**Explanation:** "Shouldn’t have been listening" is used to indicate that it was wrong or inappropriate for someone to have been listening.

**Question 49:** The President______a speech, but in the end he______his mind.
- **A.** would deliver - had changed
- **B.** was going to deliver - changed
- **C.** delivered - had changed
- **D.** was delivering - changed

**Answer:** **B. was going to deliver - changed**
**Explanation:** "Was going to deliver" indicates an intention that was not fulfilled, and "changed" is the simple past tense indicating the action that happened.

**Question 50:** The Angkor complex______the entire range of Khmer art from the 9th to the 14th century.
- **A.** represented
- **B.** represents
- **C.** represent
- **D.** was represented

**Answer:** **B. represents**
**Explanation:** "Represents" is the present tense, which is appropriate because the Angkor complex still exists and continues to represent Khmer art.

I hope this helps! If you have any more questions, feel free to ask.