In FM Stereo Broadcasts, Why Are the Left and Right

Multiple Choice

In FM stereo broadcasts, why are the left and right audio signals first converted to L+R and L-R signals before being frequency modulated?

A) This permits higher intelligence frequencies to be used
B) This permits compatibility between monaural and stereo systems
C) This is necessary to be able to be multiplexed properly
D) This produces higher signal-to-noise ratios