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?

为保证司乘人员的安全，轿车上设有安全带未系提示系统．当乘客坐在座椅上时，座椅下的开关S₁闭合，若未系安全带，则开关S₂断开，仪表盘上的指示灯亮起；若系上安全带，则开关S₂闭合，指示灯熄灭．下列设计最合理的电路图是（  ）

2.7992×25得数保留一位小数约是（      ）