Which event marks the end of one menstrual cycle and the beginning of a new cycle?

Menstrual flow and ovulation

Explanation:

The menstrual cycle is the hormones regulated the complex process. In this process, endometrium (a thickened lining of uterus wall) ruptures. Rupturing of endometrium results into the menstrual fluid which contains blood, cells from the lining of the uterus and mucus. The flow of menstrual fluid from the vagina is called menstruation. This process is to prepare the woman for expected fertilization.

Menstruation marks the end of one cycle and the release of ova from one of the ovary is beginning of next menstruation that occurs almost 14 days before menstrual flow.

answer: - b cells

explanation: -

so, the b cells mediate their action by identifying the antigens present on the surface of the pathogens and then producing antibodies in return to mediate its effect.