Correct option is B
A comparator is an indirect type of measuring instrument.
Hydraulic systems are not used as comparators in metrology. Comparators are precision instruments used to compare a dimension of a part to a standard or master dimension. It is used to compare the dimension of workpiece with a standard size workpiece.
Comparators are classified as—
(i) Mechanical comparators
(ii) Electrical comparators
(iii) Optical comparators
(iv) Pneumatic comparators
