The depth of the water at the end of a pier changes periodically along with the movement of tides. On a particular day, low tides occur at 12:00am and 12:30pm, with a dept of 2.5 m, while high tides occur at 6:15am and 6:45pm, with a depth of 5.5 m. Let t=0 be 12:00 am. Which periodic function, since or cosine would be simpler model for the situation?