Assume a Gram of a Substance Marketed as "Pure Energy

Assume a gram of a substance marketed as "Pure Energy" is annihilated by a gram of a second substance "Anti-Pure Energy." How long would the energy released power a city which uses 10⁹ watts of power?

A) 40 hrs
B) 25 hrs
C) 50 hrs
D) 65 hrs
E) 50000 hrs