6. The overall isothermal efficiency of compressor is defined as the ratio of (a) isothermal h.p. to the BHP of motor (b) isothermal h.p. to adiabatic h.p. (c) power to drive compressor to isothermal h.p. (d) work to compress air isothermally to work for actual compression (e) isothermal work to ideal work. 7. The most efficient method of compressing air is to compress it (a) isothermally (b) adiabatically (c) isentropically