As is known to all, theory is based on practice and _____ serves practice.

A．in hand

B．in turn

C．in return

D．in exchange

德国人约翰•谷登堡约在1445年发明铅活字印刷，比中毕毕发明泥活字晚约（）年。

A、100

B、400

C、700

D、1000