Why the method of coitus interruptus not 100% efficient? Kindly answer sir/mam.

mention one negative and one positive application of amniocentesis

What is Down's Syndrome?

One of the legal method of birth control is

1. by having coitus at the time of day break

2. by a permature ejaculation during coitus

3. abortion by taking an appropriate madicine

4. by abstaining from coitus from day 10 to 17 of menstrual cycle

Please answer with reason.