What Is the Lower Bound of a European Call on a Futures

What is the lower bound of a European call on a futures where f₀ is the futures price and X is the exercise price? Assume f₀ is greater than X.

A) the difference between f₀ and X
B) zero
C) the present value of the difference between f₀ and X
D) the ratio of f₀ to X
E) none of the above