A recent study determined that individuals who fulfill the diagnostic criteria for depression have lower levels of blood oxygen saturation than do individuals who are not clinically depressed. The researchers who performed this study concluded that normal levels of blood oxygen saturation guard against the development of clinical depression.

The experimenters' conclusion depends on which of the following assumptions?

A Clinical depression does not cause individuals' levels of blood oxygen saturation to decrease.
B Individuals with clinical depression experience the same physical and mental symptoms as do individuals with a decreased supply of oxygen.
C High levels of blood oxygen saturation guard against the development of clinical depression more effectively than normal levels of blood oxygen saturation do.
D Individuals with normal levels of blood oxygen saturation cannot develop clinical depression.
E Treatments for blood oxygen saturation are more effective than treatments for clinical depression.