In 1978, nearly 30% of children aged 5 to 10 years had dangerously elevated levels of lead in their blood, which can cause brain damage. What was the rate in 2001?

According to the discussion of child-directed speech,

a. adults who use child-directed speech tend to make language acquisition somewhat easier for children. b. adults who use child-directed speech actually make language acquisition more difficult for children. c. unfortunately, fathers are not skilled in using child-directed speech. d. unfortunately, older children are not skilled in using child-directed speech.

Many theorists believe that important aspects of the acquisition of grammar are biologically predetermined. To determine what knowledge is innate, it has been especially helpful to design cross-linguistic studies of

a. how quickly children enter the naming explosion phase. b. whether children are referential or expressive in their language usage. c. what is hard and what is easy for children to acquire. d. the match between adults' and children's styles across many languages.