A) 2/3 ? back solve ,lucky to hit the first choice.
My advice for this forum is to NEVER backsolve. Learn the proper techniques since you are not under time constraint. Backsolve only when you are pressed for time or have already spent too long on problem.
Backsolving and substitution as primary techniques can be addicting and counterproductive.
Former Senior Instructor, Manhattan GMAT and VeritasPrep
Vice President, Midtown NYC Investment Bank, Structured Finance IT
MFE, Haas School of Business, UC Berkeley, Class of 2005
MBA, Anderson School of Management, UCLA, Class of 1993