On a vernier caliper, how do you know which mark to use on the very top scale?

Answer

To know where it starts we look where the zero mark of the vernier scale starts. The make just before reaching where the zero mark is marks the value to use.

Explanation

A vernier caliper is an instrument that is used to measure the diameter of small circular objects such as diameter of a wires, thickness of an iron sheet.

The objects to be measured is place between the jaws of the calipers.

The vernier scale has two scales, the vernier scale and the main scale which is the very top scale. To know where it starts we look where the zero mark of the vernier scale starts. The make just before reaching where the zero mark is marks the value to use.

layer a is the exosphere and layer b is the thermosphere.   is the answer i think but i hope this .