一种格纸，每张有804格，有一篇文章大约写了20张，这篇文章大约有多少个字？
804×20≈16000（个）．
答：这篇文章大约有16000个字．
Several days has passed _______ the air crash (空难) happened. [ ]
A. since
B. after
C. as
将活塞顶的燃气（）转变为曲轴的转矩，输出机械能。
A、空气
B、压力
C、油分子
