Answer:D
Explanation: The pulmonary air sacs of birds only permit a unidirectional flow of air through the lungs. Unidirectional flow means that air is inhale and exhale through one trachea alternatively. This means the bird lungs only receive largely fresh air which has a high oxygen content during inhalation and do not receive fresh during exhalation. While air flow bidirectional in mammals.
Answer:
Genotype of long winged fly is Ll
Genotype of short winged fly is ll
Explanation:
Given
Allele for long wing trait is "L"
Allele for short wing trait is "l"
When a long winged fly is crossed with a short winged fly, half of the offspring have long wings and other half have short wings as the ratio is 
Let us say that the genotype of long winged fly is Ll
and the genotype of short winged fly is ll
The cross between the two will produce following offspring
Ll * ll
Ll, Ll, ll, ll
Out of four off spring two have long wings (Ll)
and the other two have short wings (ll)
Thus the ration of long to short wing offspring is
i.e 
Through Polymerization larger molecules are formed.
The answer would be appropriately shaped receptors
During prophase, the complex of DNA and proteins contained in the nucleus, known as chromatin, condenses. The chromatin coils and becomes increasingly compact, resulting in the formation of visible chromosomes. Chromosomes are made of a single piece of DNA that is highly organized.