患者，女性，9岁。因为上唇外伤性缺损行下唇组织瓣转移修复上唇缺损，其组织瓣成活后断蒂的时间为（）。

A.术后一周

B.术后三周

C.术后十天

D.术后一个月

E.术后两个月

Air pollution is caused by many things. Great clouds of smoke come from factories. Cars send smoke and fumes into the air. Burning waste fills the air with bits of dirt. Even burning leaves add smoke to the air. Sometimes, planes cannot find their landing spots through smoke. Smoke of all kinds is bad to breathe and can do great harm to the body.
Still, we cannot live without air. We must find a way to clean the air we have.
In time, we may have factories that are run by atomic energy. Our cars may run on smoke-free electric power. Scientists are also working on new ways to help the oil burned by cars from making fumes. Waste may be burned in the sand on ocean floors.
These changes might keep our air clean in the years to come. But until then, many scientists are looking for ways to make air cleaner now.

Scientists are now working on ways to ______.

A．keep planes from :flying

B．bury sand on ocean floors

C．stop air pollution

D．burn leaves without smoke