Question

investigations c and d use existing smokers rather than making new ones. this suggests... investigations c and d are observational studies. researchers wanted to limit variables. results were needed in a shorter timeframe. investigations c and d are controlled experiments.

Explanation:

In observational studies, researchers observe existing phenomena or groups without manipulating variables. Since Investigations C and D use existing smokers instead of creating new ones, they are likely observational studies. Controlled experiments involve manipulating variables. There's no indication time - frame is a factor or that variables are being limited in the given information.

Answer:

Investigations C and D are observational studies.