3, D (because oral epithelial cells are colorless and need to be stained);

4, c (for oral epithelial cells, isotonic solution is physiological saline);

13, b (the difference between light and shade is expressed by the density of points);

1, a (heart beating needs a lot of energy);

10, c (this is one of the reasons why cells cannot grow indefinitely-the smaller the ratio of surface area to volume, the lower the material exchange ability of cells);

7.c (leaves are vegetative organs of plants);

4.b (Stem cells are omnipotent and can differentiate into various tissues and organs)

2、B

10, b (the phage DNA is integrated into the DNA of the host cell, replicated along with the replication of the host DNA, and transcribed and translated into a new protein capsid, and the protein capsid is assembled with the newly replicated DNA to form a new phage).

5, D (small intestinal glandular epithelium mainly secretes intestinal juice and pancreatic juice, myocardium is muscle tissue, and blood is connective tissue)