If x, y, and z are positive integers and x²=y²+z², which of the following must be true ？（）
Ⅰ.x＞z
Ⅱ.x=y+z
Ⅲ. y²+z² is a positive integer

A. Ⅰ only

B. Ⅱ only

C. Ⅲ only

D. Ⅰ and Ⅱ only

E. Ⅰ and Ⅲ only

“安全第一，预防为主”是我国安全生产的基本方针。