口服四环素治疗牙周炎是因为四环素具有()。
A.龈沟液中浓度高
B.抑制胶原酶活性
C.对多种牙周炎致病菌均有抑菌作用
D.阻止骨吸收的作用
E.以上均是
参考答案:E
一颗绕地球做匀速圆周运动的人造卫星,离地高度为h.已知地球半径为R,地球表面的重力加速度为g.求人造卫星绕地球的运行周期T.
With all his attention ________ on his experiment, he has no time to spend with his family.[ ]
A.paid
B.focused
C.attracted
D.settled