5-羟色胺的前体是（ ）
A．丙氨酸
B．色氨酸
C．谷氨酸
D．苏氨酸
E．组氨酸
参考答案：B
已知a=1999，则|3a3-2a2+4a-1|-|3a3-3a2+3a-2001|=______．
用括号内所给形容词的适当形式填空。
1. Mr Read is very ______ (short).
2. She is ______ (old) of us all.
3. I think diving is ______ (difficult) activity of all.
4. English is ______ (important) than math.
5. Kitty has ______ (little) milk of the three.
