​The Single Most Important Factor in Growth of Vocabulary Is

​The single most important factor in growth of vocabulary is the child's language environment. Children have larger vocabularies when they are exposed to much high-quality language. The more words that children hear, the better. Specifically, children learn more words when their parents' speech is rich in different words and is grammatically sophisticated and when parents respond promptly and appropriately to their children's talk.  -adapted from Kail and Cavanaugh, Human Development: A Life-Span View , p. 165
Which detail below is NOT one of the ways parents can help their children develop a large vocabulary?​

A) Parents' speech includes a variety of words
B) Parents praise children when they use a new word
C) Parents use grammar that is more complex
D) Parents respond quickly when their children speak