Answer:
a) 22
Explanation:
Cardiogenesis which is the formation of the heart begins by day 21. The heart begins to form as paired tubes. By days 22 - 23 of embryonic development, the heart begins to beat. This represents the formation of the first vital organ of the body and it also causes the formation of left-right asymmetry of the body.
The days are counted from the day of the last menstrual period.
