Which of the following statements about second$-$language learning is true?

Select one:

a$.$ For adolescents and adults, new vocabulary is easier to learn than new sounds or new grammar.

b$.$ Children's ability to pronounce words with a native$-$like accent in a second language typically increases with age.

c$.$ Sensitive periods for learning a second language are constant across different language systems.

d$.$ Adults tend to learn a second language more slowly than children, but their final level of second$-$language attainment is higher.