如果x=2是方程$\frac{1}{2}$x+a=-1的解，那么a的值是 （ ）

根据短文，从A、B、C、D四个选项中选择正确答案填空。

    Mike is nine this year, but he's 1 in Grade One. He's tall in his class and his 2 is in the last row(排). Miss King usually wears 3 and can't see what the boy does there when she's near the blackboard. Sometimes Miss King asks him some 4. He stands up but doesn't know what she 5. Miss King often phones his father. The man is very busy. He 6 his wife to help Mike, but she doesn't know 7 and can't do that. She can only make him do his homework at home after supper. And she tells the boy she will buy a nice bike for him if he makes 8 in the exam(考试).

    This afternoon Mike 9 happily. When he sees his mother, he says, “I make a hundred this time.” “What?” says the woman, “What do you make it in?”“I make 10 in reading, forty in spelling and twenty in Maths.”