题目内容：

    Over a million children have developed some form of intellectual （智力） disability over the past two decades after being exposed to chemicals including flame retardants, pesticides, lead, and mercury, a study has revealed.

The study focused on the four chemicals, which can be crossed from a mother to her unborn baby through the placenta（胎盘）and cause neurodevelopmental damage. The researchers looked at data from past studies, including blood samples taken from women of childbearing age and 5-year-olds.

Over the period, flame retardants, resulted in 738,860 cases of intellectual disability and an estimated 162 million lost IQ （智商）points, followed by lead at 330,684 cases, pesticides at 111,830, and mercury at 8,860. In recent years, pesticides and flame retardants have overtaken lead and mercury as the chemicals responsible for the biggest loss of IQ among children.

The researchers believe this is due to regulations cutting down the use of these chemicals. For instance, lead has been banned from gasoline, paint and drinking water systems in the U.S. and plants are not allowed to pump out mercury.

The researchers acknowledged limitations to their study, including that the exposure-response relationships might not relate to all populations. Positively, they found a huge drop in the estimated loss of IQ points among children.

Lead researcher Abigail Gaylord, commented: "Our findings suggest that our efforts to reduce exposure to heavy metals are paying off, but that harmful exposures in general continue to represent a terrible risk to people's physical, mental, and economic health. "Unfortunately, the minimal policies in place to remove pesticides and flame retardants are clearly not enough," she said.

She also added. Frequently opening windows to let persistent chemicals found in furniture  electronics, and carpeting escape, and eating certified organic produce can educe exposure to these harmful things"

1.Which contributes most to the loss of children's IQ?

A.Flame retardants. B.Pesticide.

C.Lead. D.Mercury.

2.Where does the passage probably come from?

A.A school report. B.A science fiction.

C.A scientific research. D.A chemistry book.

3.What is the drawback of the research mentioned?

A.It has received bad response from all populations.

B.It has no previous information to refer to.

C.It covers only part of the population.

D.It is not supported by government policies.

4.What's the best title for this passage?

A.Four chemicals harming children.

B.Regulations cutting down the use of heavy metals.

C.Environment risking people's physical and mental health.

D.Children IQ lowered by exposure to harmful chemicals.