— Oh no! We’re too late. The train _______.

— That’s Ok. We’ll catch the next train to London.

A．was leaving

B．had left

C．has left

D．has been leaving

Gibbs和Emerson反应的条件是

A．酚羟基
B．有a-OH
C．有β-OH
D．有活泼氢
E．酚羟基对位有活泼氢