排水系统的排水管道由（）组成。
A．排水支管
B．通气管
C．排水立管
D．排出管
参考答案：A, C, D
—Do you know the colors of the Olympic rings?
—Yes. They're blue, yellow, black, green and red. You can find ____ one of these colors in the flags
of every country. [ ]
A. at least
B. at most
C. only
D. more than
精子和卵细胞产生后，参与繁殖过程的正确顺序是
①受精 ②肺泡植入子宫内膜 ③受精卵发育 ④排卵
A．①②③④
B．④③②①
C．④①③②
D．④②①③
