若t>0，那么$\frac{1}{2}$a+$\frac{\mathrm{}}{\mathrm{}}$t与$\frac{a}{2}$的大小关系是（       ）

显微镜中，能使物体放大的结构是    (    )

认真分析图所示的实验装置，并回答有关问题：

C

    It is a true story. Once upon a time, one of Alex's horses was stolen by his neighbor. Alex went to the neighbor's home with a policeman to get his horse back. But the neighbor refused to give the horse back. He said that it was his horse.

    Suddenly Alex had a good idea. He put both of his hands over the eyes of the horse and said to the neighbor, “If this is your horse, then you must tell us in which eye the horse is blind.”

    “In the left,” said the neighbor. Alex took his hand from the left eye of the horse and showed the policeman that the horse was not blind in the left eye.

    “Oh, I have made a mistake,” said the neighbor. “He is blind in the right eye.” Alex then showed that the horse was not blind in the right eye, either.

“I have made another mistake,” said the neighbor.

    “Yes,” said the policeman, “and you have also proved that the horse isn't yours. You must give it back to Mr. Alex. ”