When it is either not possible or unethical to manipulate an independent variable, which research method would be optimal?

In the nineteenth century, the discovery of syphilis helped scientists understand that

a) psychological disorders could be contagious and transmitted through human-to-human contact. b) the spread of bacteria caused most psychological disorders c) mental illness was caused by ruptured blood vessels in the brain. d) a physical disease could cause a psychological disorder.

From recent research, it seems that handedness begins to develop

a. at age 1 month. b. when school starts. c. before the baby is born. d. in the early teens.