Some viral diseases are dangerous only when a person is infected for the first time; subsequent infections produce only mild symptoms or go entirely unnoticed. This pattern can be explained by the action of .

1. Memory cells that trigger a secondary immune response

2. Specific antibodies that remain concentrated in the bloodstream for life after the first infection event

3. Memory cells that trigger a primary immune response

4. T cells that trigger a primary immune response

1. Memory cells that trigger a secondary immune response

Explanation:

During the first infection when the body is invaded by a particular antigen, the immune system tries to identify the type of antigen. B lymphocytes make antibodies when the antigen has been identified in the event of a primary infection. The response of the immune system during the primary infection of that pathogen is usually slow. As soon as the primary infection is arrested by the antibodies produced in response, memory cells such as the memory B cells is formed, which enables the immune system to identify the same pathogen, and thereby triggers a faster secondary immune response in subsequent infections by the same pathogen.

it's c. (to reduce the greenhouse effect/carbon gasses)

water is a polar molecule. a chemical bond is a force of attraction between atoms or ions. bonds form when atoms share or transfer valence electrons. atoms form chemical bonds to achieve a full outer energy level, which is the most stable arrangement of electrons. so, false

explanation:

in h2o molecule, two water molecules are bonded by a hydrogen bond but the bond between two h - o bonds within a water molecule are covalent. the greater the electronegativity difference, the more ionic the bond is. bonds that are partly ionic are called polar covalent bonds. nonpolar covalent bonds, with equal sharing of the bond electrons, arise when the electronegativities of the two atoms are equal.