When the lower temperature of a refrigerating machine is fixed, then the coefficient of performance can be improved by

Nusselt number (N_N) is given by

A. $${{\text{N}}_{\text{N}}} = \frac{{{\text{h}}l}}{{\text{k}}}$$

B. $${{\text{N}}_{\text{N}}} = \frac{{\mu {{\text{c}}_{\text{p}}}}}{{\text{k}}}$$

C. $${{\text{N}}_{\text{N}}} = \frac{{\rho {\text{V}}l}}{\mu }$$

D. $${{\text{N}}_{\text{N}}} = \frac{{{{\text{V}}^2}}}{{{\text{t}}{{\text{c}}_{\text{p}}}}}$$

In case of sensible heating of air, the coil efficiency is given by (where B.P.F. = Bypass factor)

A. B.P.F. - 1

B. 1 - B.P.F.

C. $$\frac{1}{{{\text{B}}{\text{.P}}{\text{.F}}{\text{.}}}}$$

D. 1 + B.P.F.

The undesirable property of a refrigerant is

A. Non-toxic

B. Non-flammable

C. Non-explosive

D. High boiling point

The desirable property of a refrigerant is

A. Low boiling point

B. High critical temperature

C. High latent heat of vaporisation

D. All of these