Read the introduction from a geology book.

Metamorphic rocks are rocks which have changed from one type to another. For example, slate is a metamorphic rock made by clay. Scientists believe that heat and pressure are two important elements in changing rocks. The following experiment with aluminum foil will help prove this theory.

Which is the hypothesis found in this introduction?

A. Metamorphic rocks are rocks which have changed.
B. Slate is a metamorphic rock made by clay.
C. An experiment with aluminum foil will prove a theory.
D. Heat and pressure change rocks from one type to another.