问题
选择题
To help Tommy learn better, his parents have done they could：cards,
tapes，special learning centers，in short, everything they can think of．
A both B all C none D neither
答案
答案：B
此处考查不定代词。由下文的cards,tapes，special learning centers可知，此处是指三者或三者以上的事情。Both意味两者都；none意为三者或三者以上的都不；neither意为两者都不。
，则小球对管内壁无压力
，则小球对管内上壁有压力
