Beijing ______a lot in the past ten years. Now it has become one of the biggest modern______ in the world.

A．changes, cities

B．changed, city

C．changed, cities

D．has changed, cities

Giemsa染色法进行螺旋体染色时，放入Giemsa染色工作液中的时间是（）

A.1小时

B.2小时

C.4小时

D.6小时

E.8～18小时或更长