Here are the closest meanings for each of the questions:

**Question 46.** The air here is too polluted for us to breathe.
**Answer:** A. The air here is so polluted that we can't breathe.

**Question 47.** You don’t try to work hard. You will fail in the exam.
**Answer:** A. Unless you don't try to work hard, you will fail in the exam.

**Question 48.** "Let's go out for a walk now," he suggested.
**Answer:** B. He suggested to go out for a walk then.

**Question 49.** This is the first time I have attended such an enjoyable birthday party.
**Answer:** B. I had the first enjoyable birthday party.

**Question 50.** We should turn off the lights before leaving the room.
**Answer:** D. The lights should be turned off before we leave the room.