In a random sample of 40 children, the mean age at which they first began to combine words was 16 months, with a standard deviation of 9.6 months. Construct a 96% confidence interval for the mean age
a) ( 16 ± 1.96 ) months
b) ( 16 ± 2.05 ) months
c) ( 16 ± 1.64 ) months
d) ( 16 ± 2.58 ) months