a. babbling 1 month, receptive vocabulary 5 months, speaks first word 8 months, speaks 2-3 word sentences 12 months
b. babbling 1 months, receptive vocabulary 4 months, speaks first word 7 months, speaks 2-3 word sentences 18 months
c. babbling 3 months, receptive vocabulary 6 months, speaks first word 12 months, speaks 2-3 word sentences 18 months
d. babbling 3 months, receptive vocabulary 10 months, speaks first word 18 months, speaks 2-3 word sentences 20 months