Which of the following is wrong with regard to longitudinal research studies?

(A) The study population is visited a number of times at regular intervals.

(B) There is no need to visit more than once.

(C) Intervals might be as short as a week or longer than a year.

(D) Intervals must be longer than a year.

(E) Intervals are not fixed, so their length may vary from study to study.

Choose the correct answer from the options given below: