臭氧层位于大气圈的()
A.对流层
B.平流层
C.中间层
D.暖层
参考答案:B
Having been out of touch with his parents for two years, John returned home,
A.safe and sound
B.safely and sound
C.safe and soundly
D.safely and soundly
在一个正常女性的体细胞中含有 [ ]
A.两个X染色体
B.一个X染色体一个Y染色体
C.两个Y染色体
D.一个X染色体