I don't know who invented the machine,but I think it is a/the useful invention.a还是the?

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Idon''tknowwhoinventedthemachine,butIthinkitisa/theusefulinvention.a还是the?Idon''tknowwhoinventedthemac

I don't know who invented the machine,but I think it is a/the useful invention.a还是the?
I don't know who invented the machine,but I think it is a/the useful invention.a还是the?

I don't know who invented the machine,but I think it is a/the useful invention.a还是the?
a 意为“一种”,表示泛指.