In a counter flow heat exchanger,cold fluid enters at $$30^\circ C$$ andleaves at $$50^\circ C$$, whereas the hot fluid enters at $$150^\circ C$$ and leaves at $$130^\circ C$$. The mean temperature difference for this case is

A

$$20^\circ C$$

B

$$80^\circ C$$

C

$$100^\circ C$$

D

$$120^\circ C$$