What should not be used to support a scaffold because they could collapse, sending the structure to the ground?

Cinder blocks

Explanation:

From safety training guides, whenever there is inadequate support, poor

construction, or a movement in the components of the scaffold such as the coupler as well as the base upon which the scaffold structure is built, there will be a great danger of collapse.

In light of this safety rule, Cinder blocks such as sandcrete blocks, bricks e.t.c shouldn't be used to support scaffold because they have a high tendency of being crushed under load

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.