When the head pumped against is less than the head of the fluid used for pumping, the usual device is a/an

A. Thermal conductivity B. Electrical conductivity C. Specific gravity D. Electrical resistivity

A. $$\frac{{\text{V}}}{{{{\text{V}}_{\max }}}} = {\left( {\frac{{\text{x}}}{{\text{r}}}} \right)^{\frac{1}{7}}}$$ B. $$\frac{{\text{V}}}{{{{\text{V}}_{\max }}}} = {\left( {\frac{{\text{r}}}{{\text{x}}}} \right)^{\frac{1}{7}}}$$ C. $$\frac{{\text{V}}}{{{{\text{V}}_{\max }}}} = {\left( {{\text{x}} \times {\text{r}}} \right)^{\frac{1}{7}}}$$ D. None of these

A. d B. $$\frac{1}{{\text{d}}}$$ C. $$\sigma $$ D. $$\frac{l}{\sigma }$$

A. $$\frac{{4\pi {\text{g}}}}{3}$$ B. $$\frac{{0.01\pi {\text{gH}}}}{4}$$ C. $$\frac{{0.01\pi {\text{gH}}}}{8}$$ D. $$\frac{{0.04\pi {\text{gH}}}}{3}$$