Mr. Irvine, who ________ in a high school for forty years, is now living in retirement.

A．had taught

B．taught

C．has taught

D．had been teaching

欧式权证的持有人只有在约定的到期日才有权买卖标的证券，而美式权证的持有人在到期日前的任意时刻都有权买卖标的证券。( )