土壤一旦被重金属污染，是较难彻底消除的，对人类危害严重。( )
参考答案：对
I haven't seen Anna since she left our hometown two years ago, but we do keep in touch with each other
by sending emails ________.[ ]
A．here and there
B．sooner or later
C．at one time
D．now and then
"月经"之名最早见于（）
A.《黄帝内经》
B.《金匮要略》
C.《脉经》
D.《难经》
E.《医贯》
