I cannot______ puzzled, for I simply cannot understand how I could have made such a stupid mistake

Mr Green has two children. One is a son, and ______ is a daughter.

The thread of my kite broke and it flew away. — I told you it would easily break_____ it was the weakest.[ ]

There were two buildings, _______ stands nearly a hundred feet high.

________ I am concerned, I think the project should be held off right now.