Question

read the sentence from the evolution of useful things
making waterproof sandpaper would allow wet sanding, which in turn would cut down on dust, and thus be a great improvement.
the underlined phrases indicate
passing of time
a directional change of position
a cause - and - effect relationship
sequence of events

Explanation:

The sentence shows that making waterproof sandpaper (cause) leads to wet sanding, which then leads to less dust and a great improvement (effects). The phrases "which in turn" and "thus" show a cause - effect link. "Passing of time" is not indicated. "Directional change of position" is irrelevant. "Sequence of events" is too general; this is specifically cause - effect.

Answer:

a cause - and - effect relationship