Which equation can be used to find the answer?

After two months, Rachel has saved enough money to buy an MP3 player. During the second month, she saved $87 and she pays $199 for the MP3 player.

How much did Rachel save during the first month?

Am = 87 • 2

m = 174

$174

B199 + 87 = m

m = 286

$286

Cm + 87 = 199

m = 112

$112

Dm – 87 = 199

m = 287

$287