Why Do Natural Methods of Contraception Have High Failure Rates

Why do natural methods of contraception have high failure rates?

A) Determining the time of ovulation is a complicated process which depends on many factors.
B) The frequency of ovulation is increases and this leads to higher chances of conception.
C) There are only a few days in a month when a couple can have sex without the risk of pregnancy.
D) These methods increase the chances of contracting an STI.
E) It completely stops ovulation and disrupts a woman's menstrual cycle.