I found in a question paper the following questions which is not clear for me. Pls explain.

1. Identify the sentence with the correct question tag:

a. In that case he has to go now, hasn't he?
b. In that case they cannot help me, will they?

My choice is (a) but the answer given was (b)

2. Fill in the blanks suitable

I ______________ Mrs. X, but I ________ that she ________ not here today.

a. came to see, understood, is
b. came to see, understood, was
c. came to see, understand, is
d.came to see, have understood, was

The answer given was (c) but why not (b)

3. We desire to reap, therefore we sow. (Identify the sentence)

a. Simple b. Compound c. Complex d. None of these

The answer given was (b) but i have read that compound sentence has the clue i.e conjunction (FANBOYS) in the sentence but here there is no such word. How?