For many experiments, it is desirable to have a population of cells that are traversing the cell cycle synchronously. One of the first, and still often used, methods for synchronizing cells is the so-called doubled thymidine block. When high concentrations of thymidine are added to the culture fluid, cells in S-phase stops DNA synthesis, though other cells are not affected. The excess thymidine blocks the enzyme ribonucleotide reductase, which is responsible for converting ribonucleotides into deoxyribonucleotides. When this enzyme is inhibited, the supply of deoxyribonucleotides falls and DNA synthesis stops. When the excess thymidine is removed by changing the medium, the supply of deoxyribonucleotides rises and DNA synthesis resumes normally.

For a cell line with a 22 h cell cycle divided so that M phase= 0.5 hour, G1 phase= 10.5 hours, S phase= 7 hours, and G2 phase= 4 hours, a typical protocol for synchronization by a double thymidine block would be as follow:
at 0 hours ( hours) add excess thymidine
after 18 hours ( hours) remove excess thymidine
after an additional 10 hours ( hours) add excess thymidine
after an additional 16 hours ( hours) remove excess thymidine

A) at what point in the cell cycle is the cell population when the second thymidine block is removed?

B) Explain how the times of addition and removal of excess thymidine synchronize the cell population