Suppose That the Demand Curves for Goods A, B, and C

Suppose that the demand curves for goods A, B, and C have the following functional forms:, where Q denotes quantity demanded and P denotes price:
QA = 120 - 3.5PA - 6PB QB = 100 - 2PB + 3PC QC = 1500 - 0.5PC.
Based on these demand curves, which of the following pairs of goods are known to be substitutes?

A) A and C
B) A and B
C) B and C
D) A and C, and B and C
E) none of the pairs are substitutes