孔子曾说：“中上之人，可以语上（深奥的学问）也；中下之人，不可语上也。”这表明了哪一教学主张？（  ）

A．有教无类

B．启发诱导

C．温故知新

D．因材施教

_______ is known to us all that the old scientist, for _______ life was hard in the past, still works hard in his eighties. 

A．It; whom

B．As; whose

C．As; whom

D．It; whose