They provide the immune system with "memory" against previously encountered pathogens.
T memory cells are necessary for protective immunity against invading pathogens
Answer:
True
Explanation:
The Devonian Period comprises the years between 416 million and 354 million and is subdivided by periods: Lower (the oldest), Middle and Upper (the most recent).
During this period, the main transformations occurred in the flora, with the exponential growth of small terrestrial plants through the development of spores. With this process, plants will become more similar with modern plants. During this period the plants were able to fertilize themselves with the seeds and reach the height of trees. They formed the first forests to be recorded, with the appearance of licopodia, ferns and progimosperms.
Water is a very polar molecule.
This means there is a (slight) difference in charge between the ends of the water molecule. Water is made up of an Oxygen and two Hydrogens (H₂O). The Oxygen is a much larger atom, with 8 protons in its nucleus, whereas Hydrogen has only 1 proton.
As protons are positively charged, the oxygen has a greater positive charged, so the electrons which are negatively charged are more attracted to the Oxygen, and the bond shifts slightly towards the oxygen. As the electrons have shifted position, the negative charge has shifted position, to there is a slight negative charge nearer the Oxygen, δ⁻ (delta minus), and a slight positive charge, δ⁺ (delta plus), at the Hydrogen.
This allows strong inter molecular forces to occur, as the slightly positive Hydrogens are attracted to the slightly negative Oxygens. This force of attraction between water molecules causes them to want to 'stick together', causing a high surface tension.
So the answer is B)
Explanation:
1)genetic drift- stochastic variation in inheritance
2)assortative mating
3)mutation
4)Natural selection
5)migration (gene flow)
