One of my favorite Quantitative Comparison strategies is called "Looking for Equality" (free video on this topic here: http://www.greenlighttestprep.com/modul ... on?id=1099).
In this strategy, I first look for values of x and y that make the two quantities are equal. This often involves checking some easy numbers like 0, 1, -1 etc.

Consider these two cases:
Case #1: x = 0 and y = 0
Quantity A: (x+y) = 0+0 = 0
Quantity B: 2(x+y) = 2(0+0) = 0
Since the two quantities are EQUAL, the correct answer is EITHER C or D

Now try some other values of x and y:
Case #2: x = 1 and y = 1
Quantity A: (x+y) = 1+1 = 2
Quantity B: 2(x+y) = 2(1+1) = 4
Since the two quantities are NOT EQUAL, the correct answer is D

Cheers,
Brent