Here are the answers to your questions along with clear explanations:

**Question 5:** Every year, heavy rains affect the central-eastern provinces of Vietnam and cause ...........
- **A. floods**
Explanation: "Floods" are a common consequence of heavy rains, as they lead to excessive water accumulation in areas. "Droughts," "earthquakes," and "eruptions" are not directly linked to heavy rains.

**Question 6:** I saw a beautiful bird while I ............... in the park this morning.
- **D. was walking**
Explanation: The past continuous tense "was walking" is used to indicate an ongoing action in the past when another action (seeing the bird) occurred. "Walked," "have walked," and "had walked" do not fit this context.

**Question 7:** During Tet holiday, many Vietnamese families visit pagodas ............ than on other occasions.
- **B. more frequently**
Explanation: "More frequently" is the correct adverbial form that indicates a greater frequency. "More frequent" is incorrect because it is an adjective, while "frequenter" is non-standard and "as frequently" doesn't indicate the comparison intended.

**Question 8:** I wish I .............. you but I’m too busy at the moment.
- **B. could help**
Explanation: "Could help" is the correct option as it expresses a wish about a present situation that is not possible. The other options imply past ability or intent, which doesn't fit with the current context.

**Question 9:** Shop assistant: “Would you please wait a few minutes?” Customer: “..........”
- **D. No worries. I’m in no rush.**
Explanation: This response is relaxed and indicates that the customer is willing to wait. "Of course, I will" is also acceptable, but the clarity of "No worries" matches the context of reassurance. The other options are either contradictory or awkward in this context.

**Question 10:** Planet Earth, .......... is the third planet from the Sun, is known for its diverse ecosystems and abundant natural resources.
- **D. which**
Explanation: "Which" is the correct relative pronoun used for things (in this case, "Planet Earth"). "What," "who," and "whose" do not fit grammatically in describing a planet.

**Question 11:** Many of the gases which are produced by factories are ........... to our health.
- **A. harmful**
Explanation: "Harmful" is the right adjective to describe gases that can negatively impact health. "Destructive" and "disastrous" imply physical damage rather than direct health effects, while "unreliable" doesn't fit the context.

**Question 12:** Tessa has so ....... homework that she can’t go out with us.
- **D. much**
Explanation: "Much" is used with uncountable nouns like "homework." "Many" is used for countable nouns, "plenty" implies an abundance but doesn't fit the construction here, and "little" contradicts the implication that she cannot go out.

If you have any more questions or need further clarification, feel free to ask!