I also thought it was A at first. Its what I picked in my 2min timing. But takin bout another minute after my alloted time, I figured out that its C.

Stmnt 1:

Since M<N>

6/7>4/5 Now lets say x=-2. 2/3<4/5 So A is NOT sufficient. We don't know whether x is negative or positive.

Stmnt 2: x>0. We dont know anything about m or n so we cannot determine anything from just this. M>N or M<N we have no idea.

UMB you can't say x=-3 for stmnt 2 b/c it says that x is greater than 0.

Now adding them together:

stmnt 1: established a relationship between m and n saying m<n>0. SO since x cannot be negative adding any positive value to m and then n will give us a greater value than m/n.

Try it even w/ fractions:

(4+1/10)/(5+1/10) equates out to 41/10/51/10--> 41/51 or 4.1/5.1 which is greater (just slightly) than 4/5.