By studying human genes, scientists hope to learn more about how to prevent and cure hereditary diseases.

Which word or group of words is the simple subject?

A. scientists

B. hereditary diseases

C. genes

D. human genes

When identifying the subject of the sentence, look for who or what the sentence is about. The basic sentence is going to be who (or what) is doing what? In this case, the sentence is about what the scientists are doing. Who? The scientists. What are they doing? They "hope to learn." So the subject is "scientists."