When the lower temperature of a refrigerating machine is fixed, then the coefficient of performance can be improved 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. $${{\text{N}}_{\text{N}}} = \frac{{{{\text{V}}^2}}}{{{\text{t}}{{\text{c}}_{\text{p}}}}}$$

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.

A. Non-toxic B. Non-flammable C. Non-explosive D. High boiling point

A. Low boiling point B. High critical temperature C. High latent heat of vaporisation D. All of these