Which word correctly applies the rule about adding letters to words that end in "y"?

The heart sound produced by the closing of the aortic and pulmonary valves is:

A) S4. B) S2. C) S3. D) S1.

This type of study has less validity than an RCT, but it has utility in some circumstances. The problem in this study design is that there is an increased chance that the study groups will be different

A) Cohort study B) Nonrandomized controlled trial C) Cross-sectional study D) Case series