Why is it important for there to be variety in a population as it relates evolution?

Variety leads to advancement, lack of variety would stop advancement in society.

Therefore, variety is important in a population as it leads to higher possibilities for advancement and evolution within a species.

Hope this helps!

I found the exercise on the internet and this is the statement: "We gain energy from eating plants. Plants can't create their energy; they must first get it from the Sun."

Therefore, the gaps should be filled with "true" and "first".
It is true that we get energy from the plant and their energy comes from the Sun, by using UV radiation, not from themselves. This is also in accordance with the first law of the thermodynamics - the internal energy variation of a closed system is equal to the difference of the quantity of heat that's provided to the system and the amount of work performed by it. So, the energy provided by a plant is equal to the heat that it collects minus the energy that the plant itself uses up.