A Fall in the Price of X from $6 to $4

Multiple Choice

A fall in the price of X from $6 to $4 results in an increase in the quantity of Y demanded (at the current price of Y) from 900 to 1,100 units. What is the cross elasticity of demand between X and Y?

A) 0.5
B) -0.5
C) 2
D) -2
E) Either A or B, depending on whether X and Y are substitutes or complements.