A：a2﹣b2
B：b2﹣a2
C：a2+b2
D：﹣a2﹣b2
毫无疑问，懂得一些急救知识很有必要，因为危险无处不在。
____ ____ ____ ____ that it is necessary to know some knowledge of first aid as dangers lie everywhere.
电路中有了电源，就能使（ ）
—Sure. It's on Center Street.
