已知锐角三角形的边长分别为2、4、x，试求x的取值范围（ ）
已知a，b，c，d为非负整数，则ac+bd+ad+bc=1997，则a+b+c+d=______．
The doctor recommended that you _____ swim after eating a large meal.[ ]
A. wouldn't
B. couldn't
C. needn't
D. shouldn't
