A股账户按持有人可以分为( )。

A．自然人证券账户

B．一般机构证券账户

C．证券公司自营证券账户

D．境内投资者证券账户

David’s effort was ______, because he won the first prize in the English speech contest, _____ made his classmates surprised.                                          

A．paid off; which

B．paid back; it

C．paid for; that

D．paid up; he