题目内容：
根据下列材料请回答题：Mental Retardation(智力迟钝).
Mental retardation is a condition in which people have lower than normal intelligence and are unable to function at the level expected for their age。People with mental retardation are usually born with it。or it develops early in their life.They may also have some difficulty with daily living skills such as learning to read and write and caring for them selves.
Doctors and other professionals determine that a person has mental retardation based on their intelligence and how well they can do everyday activities.
Intelligence is the ability to learn and understand.Levels of intelligence are measured by special tests called intelligence tests.The score a person gets on one of these tests gives a numerical(数字的)measure of a person’s intelligence.This is called an intelligence quotient(智商)Or IQ.
An average score on an IQ test is about 90 to 110.A person with mental retardation will usually score below 75 on an IQ test.
The IQ test alone does not determine whether someone is mentally retarded.A person must also have trouble with everyday activities such as geeing dressed，eating，washing，or learning basic reading，writing，and arithmetic skills.
Mental retardation is not a disease itself.1t cannot be cured and it’s not contagious(传染的).This condition can be caused by several things that injure the brain or don’t allow the brain to develop normally.Many times we don’t know why a person has mental retardation.
Sometimes it may be caused by genes.Genes are chemical units found in every cell They carry the instructions telling , cells what to do.Sometimes。children receive abnormal genes from their parents.
A defective gene may also develop spontaneously.Neither parent would have passed on the gene，but the gene changes before the baby develops.
Some other problems that can cause mental retardation also happen before a child is born.It is important for the brain to develop properly if a child is to have normal intelligence.“Planning a pregnancy is the most important decision most of us will make in our lifetime.A healthy lifestyle and good medical care should begin before a woman becomes pregnant and continue throughout the pregnancy，”says Jodi Rucquoi，a genetic counselor from Connecticut.If a woman abuses alcohol or drugs or doesn’t eat well，there is a risk to the developing baby.Also，a premature birth or problems during childbirth can sometimes harm the baby’s developing brain.while premature babies are generally fine，there is a greater chance that they may have mental retardation.
In some cases.a young child can develop mental retardation after being sick with a serious infection or other illness.or after suffering a bad head injury.A child can be taken as mentally retarded.f he or she. A．was born prematurel
B．has lower than normal intelligenc
C．has low intelligence and low daily living skill
D．has normal intelligence but difficulties in learnin
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