A Parent Says,"I Am Worried About My 13-Year-Old Son

A parent says,"I am worried about my 13-year-old son.He hasn't started puberty,and my daughter did when she was 11 years old." The health-care worker should explain to this parent that this is:

A) unusual and requires further evaluation.
B) unusual because the onset of puberty is usually the same in siblings.
C) normal because the onset of puberty is usually earlier in girls than in boys.
D) abnormal because the onset of puberty is usually earlier in boys than in girls.