Why is it impossible from an individual to inherit a recessive condition if only one parent is a carrier for that condition

A recessive genotype needs to be homozygous (Both alleles the same) for the phenotype to show. The common school example of course being Eye colour of Blue or Brown. If someone is a carrier, then that means they have the allele needed for the condition, but they don't show it because they are heterozygous for it, so the dominant allele shows instead. Because only person carries the recessive allele, when they reproduce, there will be no recessive allele in the second person to connect with the recessive allele in the first person, so at best the offspring can only be a carrier too. 

The objects do not reach terminal velocity         there is not enough time from the time the object falls from when it hits the ground to achieve terminal velocity. an object must be placed higher to achieve terminal velocity in order to be in free fall.

Ibelieve the answer is d. densityhope my answer has you : )