Blood cells are produced in which system?
a. circulatory system
b. immune system
c. skeletal system
d. endocrine system
user:
heart disease includes all of the following risk factors except
a. high blood pressure
b. high cholesterol levels
c. cigarette smoking
d. regular exercise

C. skeletal system
D. regular exercises

Mitosis doubles the numbers of cells, and since there are 4 five minute groups in 20 minutes, the cell would double 4 times. this can be seen as 2^n where n=number of times mitosis occurs.  if we plug in 4, we get 2^4 or 2*2*2*2 which gives us 16, so C.16