C. 6.6
A. Is not a possibility because the top line is 4.4 "x" is way bigger so that will be cancelled out.
D. Is also not a possibility as it has the same measurement as the line above but line "x" is again bigger, so cancel out D.
You now have B and C left.
If line 'x' was B, the line would be 2x the size of the top line but line 'x' isn't, so the measurement would be C.