prep-36

36.  Scientists at the Los Alamos National Laboratory have succeeded for the first time in mining heat from the Earth’s interior and producing energy on a commercial scale, enough for efficient generation of electricity and heating factories and homes.

A. and producing energy on a commercial scale, enough for efficient generation of electricity and heating

B. and producing enough energy on a commercial scale for electricity to be generated efficiently and to heat

C. for energy production on a commercial scale, enough for generating electricity efficiently and to heat

D. to produce energy on a commercial scale, enough for generating electricity efficiently and for heating

E. to produce enough energy on a commercial scale for efficient generation of electricity and heat

the answer is D

I'm quite uncomfortable with the "enough for" in answer D...any NN could help to explain the usage? thanks.