The foreigners arrived _____ Beijing two weeks ago. [ ]
A. in
B. at
C. to
D. on
A
早期胃癌的概念是指()
A.病变小于2cm
B.病变仅侵及黏膜层
C.病变仅侵及黏膜或黏膜下层
D.病变局限在胃壁内
E.以上都正确
如图所示,物体在水平面上做匀速直线运动,拉力F=60N,该装置的机械效率为70%,则物体受到的摩擦力大小是 ( )
A.60 N
B.126N
C.180N
D.257.1 N