Gender is to ____ as sex is to ____.

The lens is the:

a. transparent and protective outer covering of the eye b. light-sensitive surface at the back of the eye c. flexible piece of tissue that helps focus light toward the back of the eye d. ring of colored tissue around the opening in the center of the eye

Using an ABA reversal design is rather like using a ______

a. dust mop b. fountain pen c. light switch d. barber's chair