Here's a problem to solve: It was once thought that living organisms could come from non-living matter. For example, people believed that flies would develop from rotting meat. This idea was later disproved primarily because of

Among the 4 following options, which is the correct answer?

- A: the discovery of the atom.
 
- B: better surgical techniques.
 
- C: continued experimentation.
 
- D: the invention of the microscope.
 