I’m sorry I delayed_________ your letter because I have been quite busy these days.

A．to answer

B．to have answered

C．answering

D．answered

全口义齿中影响口腔发音的因素有（）。

A.前牙长短

B.前牙的唇舌向位置

C.腭侧基托形态

D.覆覆盖

E.以上都对