If a study categorizes subjects on the basis of some naturally occurring difference, it is most likely a(n)

Current estimates indicate that about ______ percent of people over age 65 suffer from Alzheimer's disease

a. 10 c. 50 b. 30 d. 70

On the DNA ladder, adenine always appears

A) alone. B) with thymine. C) with cytosine. D) with guanine.