The correct answers are:
A. The southwest was covered with sand dunes. This period increased aridity in the southwest of the USA
.
D. Much of the northern United States was ice covered. During the last glacial period, the north of USA was covered by ice, in especial during the Last Glacial Maximum (LGM)
.
These answers are not correct:
B. Canada was covered with ice thousands of feet thick. During this period, Canada was almost completely covered in ice (97%); however, the ice was not so thick
.
C. Florida received heavy rains. Florida was drier during this period.
The correct answer is option b, that is, a polygenic trait that is affected by the number of alleles present.
There are three genes associated with a usual form of glaucoma, so it must be the polygenic trait, that is, it comprises more than one gene. The polygenic traits are affected by the number of alleles. In this circumstance, there are more than twenty genetic loci present, which signify that there must be a larger quantity of alleles for all the loci.