Which of the following is the closest to the length of side AC?

We just started the introduction into circles and i have no idea how to do this.

Scores on a university exam are normally distributed with a mean of 78 and a standard deviation of 8. the professor teaching the class declares that a score of 70 or higher is required for a grade of at least “c.” using the 68-95-99.7 rule, what percentage of students failed to earn a grade of at least “c”?

Yvaries inversely as x. y =12 when x=5. find y when x=4