Why has two different alleles for the same trait?

Because is Heterozygous example Aa

i think it is true in my reasonment ..

explanation:

for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.

the answer is d p: cell, q: organ, r: organ system, s: tissue