题目

One day last November, Tom Baker stopped out of his house into the morning light and headed across the rice fields toward the bank of the Rapti River. Tom, a 32-year-old school teacher in the farming village of Madanpur, was going for his morning bath. As he approached(走近) the river, the head of a tiger(老虎) suddenly appeared over the edge of the river bank. Before he could turn to run, the tiger was upon him. It jumped on his shoulder and threw him to the ground, its huge jaws attacked his head in a killing bite. Peter Smith was also on his way to the river and saw the attack. He screa如图所示，在矩形ABCD中，DE⊥AC于E，设∠ADE=a，则cosa=，AB=4，那么AD的长为 [     ]A.3B.C.D.