Some teachers often give their students too much work to do, ________that they will thus improve

their school grades.

A. hoping  

B. to hope  

C. hope  

D. hoped

可用水蒸气蒸馏法提取的香豆素成分是

A．游离小分子简单香豆素

B．香豆素苷

C．呋喃香豆素

D．吡喃香豆素

E．双香豆素