用所给词的适当形式填空

①To make a sandwich, we need two ____ (slice) of bread, some vegetables and some meat.

② It rained hard yesterday, but ____ (luck) we brought our raincoats and didn't get wet.

③Ted told us an unusual story ____ (call) Mystery steps.

④ Cindy had fun ____ (make) flashcards with her cousin.

⑤ How old was Laura when she became a ____ (skate) champion?

（Ⅰ）展开式中二项系数最大项；

（Ⅱ）若 ${\left(x+2\right)}^n=a_0+a_1\left(x+1\right)+a_2{\left(x+1\right)}^2$   $+\cdots +a_n{\left(x+1\right)}^n$ ，求① $a_1+a_2+\cdots +a_n$ 的值；② $a_1+2a_2+\cdots +n{a}_n$ 的值．