氧是煤中的()元素。
A.助燃
B.有害
C.无害
参考答案:C
Although he in America for a long time, he’s never regretted his making his mind to return to his own country.
A.stayed
B.was staying
C.has stayed
D.had stayed
在互换性问题中,( )为初期所使用的一个互换性判定指数。
A.引射系数 B.核准基数 C.比例常数 D.华白数