女性，53岁。近1年先后更换3副全口义齿，均出现戴义齿后不适，咀嚼困难，义齿脱落。查：上下唇前突，闭合困难，说话时面部表情不自然。上下牙槽嵴丰满，颌间距离短。造成患者不适的主要原因是（）

A.适应能力差

B.垂直距离过高

C.上下前牙排列偏向唇侧

D.心理因素影响

E.要求过高

--The duty of Project Hope is to help poor children, isn't it?

--Yes, it has built many schools ____ those children can study happily.[ ]

A. where          

B. when          

C. which          

D. what