Which of the following best describes the concept of marginal probability?

A) It is a measure of the likelihood that a particular event will occur,regardless of whether another event occurs.
B) It is a measure of the likelihood that a particular event will occur,if another event has already occurred.
C) It is a measure of the likelihood of the simultaneous occurrence of two or more events.
D) None of these choices.

According to the Empirical Rule,if the data form a bell shaped normal distribution approximately ____________________ percent of the observations will be contained within 1 standard deviation around the mean.