People with Williams syndrome, a rare mental condition, are often highly articulate and sensitive. Not uncommonly, they are gifted in music and possess rich vocabularies. Yet these same people, because of their lack of ability in basic arithmetic and difficulty in distinguishing left from right, are misleadingly labeled mentally retarded. As evaluated by conventional means such as IQ tests, their intelligence is no higher than that of people with Down's syndrome, despite the fact that people with Down's syndrome have uniformly limited cognitive abilities and show no specialized aptitudes.

The author is arguing that

A. conventional methods of measuring intelligence, such as IQ tests, are inadequate for evaluating people with certain mental conditions such as Williams syndrome
B. people with Down's syndrome usually have less verbal and musical ability, but more mathematical and spatial ability, than do people with Williams syndrome
C. conventional methods of measuring intelligence tend to consider basic mathematical and spatial ability to be more important than verbal and musical skills
D. people with Williams syndrome are only rarely given the opportunity to develop their unique musical and verbal abilities
E. people with Williams syndrome need greater encouragement if they are to develop their mathematical and spatial skills

i am confused between A and C.

But i think A is more powerful answer. if its correct i will write my view.

A
are misleadingly labeled mentally retarded. As evaluated by conventional means such as IQ tests, their intelligence is no higher than that of people with Down's syndrome,

Will vouch for 'A'