Name the part of the vernier callipers which is used to measure the following :
(a) external diameter of a tube,
(b) internal diameter of a mug,
(c) depth of a small bottle,
(d) thickness of a pencil.
Name the instrument which can measure accurately the following :
(a) the diameter of a needle,
(b) the thickness of a paper,
(c) the internal diameter of the neck of a water bottle,
(d) the diameter of a pencil.