The current maternal blood-screening test is called the triple screen because:

The current maternal blood-screening test is also known as the triple screen test because of the testing of the three substances in the blood of a pregnant woman. These three tested components are the hCG (Human Chorionic Gonadotropin), AFP (Alpha-feto protein), and the estriol.

For the purpose of this triple screen test, the blood is drawn for the expectant mother and then this drawn blood is tested for the listed three components.

Hence, the correct answer is 'it tests for three substances in the maternal blood'.

the correct answer is - d. china has a large and growing population.

the fact that china has a large (the biggest) population, and it is still growing, doesn't hinder the agriculture production in the country. in fact, it can for increased production in the agriculture, since there's more labor force, as well the sheer mass of people produces lots of scientists that come up with new inventions as to how to improve this sector. nice example is the inner mongolia region in china, in general dominated by the gobi desert, but few scientist came up with a nice idea as to how to make the region productive by using plants that are very resistant to droughts, so lots of people have been employed, and the region is becoming greener and greener each year, and the production of food in this region is also increasing.