Clarisse is introduced prior to Mildred to create contrast, for all three characters (Clarisse, Mildred, and Montag) are different. To one extreme you have Clarisse, who is a free thinker, which in this society, will lead to her demise. She is also young and innocent.
It's most likely A. because back then women were treated like common house hold objects; they were used and when they couldn't be used anymore thrown out.
