which of the following best describes the time value of an option?

(a) time value of an option is negative if the option value is lower than its intrinsic value

(b) time value of an option equals to zero at expiration

(c) time value of an option is always positive

The correct answer is (b) but I would like to know why (a) and (c) are incorrect. I would also like to know if the option value is always greater than its intrinsic value and how can time value of an option be negative?