运输带头轮包胶的目的是（）
A、增大摩擦
B、扩大头轮
C、防止划皮带
参考答案：B
He ______ this dictionary for 5 years. He ______ it in 2004. [ ]
A. has bought; bought
B. bought; bought
C. has kept; has bought
D. has had; bought
泡性结角膜炎的病因是（）
A.细菌
B.病毒
C.衣原体
D.速发过敏反应
E.迟发过敏反应
