what‘s a plane 还是what's the plane?

what‘s a plane 还是what's the plane?
1.what（）a plane?——It’s a machine that can fly
用a还是the?
2.Do you know who invited （）telephone?——Bell,It‘s（）most useful invitation.用a还用the
麻烦把解析写出来
1.what’s（）a plane？——It’s a machine that can fly
用a还是the？
第二题答案上用a，弄不懂诶