Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

Embryos usually don't survive if they have extra or too few of these genetic units?
Embryos usually don't survive if they have extra or too few of these genetic units?

The sun and the moon appear to be about the same size in the sky because the moon is smaller in diameter and is closer to earth than the sun.
Which statement best explains why the Sun and the Moon appear to be about the same size in the sky?

A tracer is the name for a substance that can be used to follow the pathway of that substance through some structure.
What is the name for a substance that can be used to follow the pathway of that substance through some structure?