每个人都对人口问题很关注。
2．为什么会有巨大的人口增长呢
3．要改变环境，人们应采取措施。

A．viruses are known to cause cancer in animals
B．at present, human cancer is not believed to be contagious (传染的)
C．there are many known causes for the transformation of a normal cell to a cancer cell
D．results of experiments on plants and animals do not vary greatly from species tn species

A．to whether they could learn to speak without the help of adults
B．to prove that Phrygian was the first language of mankind
C．to see whether they would speak the Egyptian language
D．to discover which language would come most naturally