Magnetic force - B.
Electrical force - A.
Gravitational force - D.
Contact force - C.
<h3><u>Explanation:</u></h3>
When a charge moves about its path, it produces a magnetic force around it. This magnetic field is perpendicular to the axis of movement of the charge and is directly proportional to the amount of charge, speed of the charge, and inversely proportional to distance between the charge and the point of measurement. Michael Faraday discovered this phenomenon.
Electrical force of attraction or repulsion is seen between the two charged bodies, or one charged body and one uncharged body which can induce charge to the uncharged body. The attraction or repulsion between the charged bodies are seen if the bodies are close enough, and this is called electrical force.
The gravitational force is also a non contact force that is seen between two bodies which is always attractive in nature and is seen between any two bodies.
A contact force like friction is a type of force that takes place only when two bodies are in contact with each other.
Answer:
Because natural selection selects for it. Thus it persists.
Explanation:
You have to understand heterozygote advantage. Basically, it's where heterozygotes have an advantage over homozygotes. In the case of sickle cell disease, heterozygotes have an advantage, and natural selection favors whatever is advantageous. Thus, because heterozygotes each have one recessive sickle-cell allele, as natural selection favors the heterozygotes, the recessive sickle-cell allele persists and remains in the gene pool.
