已知x、y、z是三个非负实数,满足3x+2y+z=5,x+y﹣z=2,若S=2x+y﹣z,则S的最大值与最小值的和为( )
A:5
B:6
C:7
D:8
For now, I have to stay at home because of my illness.
青藏高原位于我国 ____ 阶梯.