31. Give an example of

(a) a physical quantity which has a unit but no dimensions
(b) a physical quantity which has neither unit nor dimensions
(c) a constant which has a unit
(d) a constant which has no unit

 
Hint: A quantity that is a ratio of similar quantities can be dimensionless or unitless.
Step 1: Find the quantities that are dimensionless or unitless.
 (a) Plane angle θ=Lr radian  its unit is radian but has no dimensions  (b) Strain =ΔLL= Change in length  length  It has neither unit nor dimensions  (c) Gravitational constant (G) =6.67×1011Nm2/kg2 (d) Reynold's number is a constant which has no unit.