A diver swims 18 metres below the surface of the sea, and the surface is taken as height 0. Select all statements that are true about the diver's signed height in metres.

Explanation

Heights are counted upwards from the surface, so a position reached by swimming downwards is recorded below zero, and eighteen metres of depth becomes eighteen below zero. Recording the plain distance throws away the direction that the sign exists to carry, and the surface itself is the only place whose height is zero.