观察下列等式:9﹣1=8,16﹣4=12,25﹣9=16,36﹣16=20,…将你找出的规律用含字母n(n为正整数)的式子表示为( )。
(n+2)2﹣n2=4(n+1)
After graduation I went to Somalia. When I first set foot on strange land, I didn't know what
the future had in store for me. [ ]
A. 不填; a
B. the; the
C. the; 不填
D. the; a
四个选项中,()是依靠阀瓣的升降以达到开闭和节流的目的,这类阀门使用方便、安全可靠,但阻力较大。
A、旋塞阀
B、截止阀
C、蝶阀
D、闸阀