Hypotheses Described in the Text, and That Have Been Proposed

Hypotheses described in the text, and that have been proposed to explain the rising wage gap between skilled and unskilled workers:

A) may be reasonable explanations for a country whose technology and participation in world markets have remained constant over the relevant time period
B) may be complementary to one another
C) must be mutually exclusive
D) are, in fact, the only possible explanations for the observed wage gap