咖啡因实际上是一种（），它主要对中枢神经系统产生作用，会刺激心脏肌肉收缩，加速心跳及呼吸。
A、兴奋剂
B、抑制剂
C、麻醉剂
D、镇静剂
参考答案：A
闭合型问题
—Last week American doctors ______ his foot.
—They say Liu Xiang may run ______ after the operation. [ ]
A. operated on; even faster
B. operation on; even fast
C. operated to; much faster
D. operated; more faster
