Answer: d. both b and c
The population structure of the United States is as follows:
Median age: 38.1 years (female: 39.4/male: 36.8)
Distribution:
0–14 years: 18.73% (male 31,255,995/female 29,919,938)
15–24 years: 13.27% (male 22,213,952/female 21,137,826)
25–54 years: 39.45% (male 64,528,673/female 64,334,499)
55–64 years: 12.91% (male 20,357,880/female 21,821,976)
65 years and over: 15.63% (male 22,678,235/female 28,376,817)
