One of these sentences contains an error in Verb Forms.
A.Historians speculate about what might have happened when the Conquistadors arrived if the Mayans had invented the wheel.
B.In the early part of the century, electric lights were an extravagance in which only the wealthiest people are indulging.
C.Arnie said that if he is going to be late for the play, he will call so that we will not waste time waiting for him.
D.After weeks of cramming for his law boards, when he sat down to take the test, he realized that his mind was as blank as a switched-off computer terminal.
The answer is B