Which statement below defines epistasis?
a. when a single gene affects several phenotypes
b. a heterozygote displays a phenotype intermediate between its two alleles
c. a measurement of phenotype intensity
d. when one locus affects or covers the outcome of another locus

 The correct assertive is letter D. When one locus affects or coverbs the outcome of another locus.

Explanation:

  Epistasis is when alleles of one gene inhibit an action of alleles of another pair, which may or may not be on the same chromosome. The gene that exerts inhibitory action is called epistatic, and the gene that inhibits it is called hypostatic. This fact about epistasis is common among animals and plants.

when it's short and dry.

the correct answer is - d. china has a large and growing population.

the fact that china has a large (the biggest) population, and it is still growing, doesn't hinder the agriculture production in the country. in fact, it can for increased production in the agriculture, since there's more labor force, as well the sheer mass of people produces lots of scientists that come up with new inventions as to how to improve this sector. nice example is the inner mongolia region in china, in general dominated by the gobi desert, but few scientist came up with a nice idea as to how to make the region productive by using plants that are very resistant to droughts, so lots of people have been employed, and the region is becoming greener and greener each year, and the production of food in this region is also increasing.