It can mean both, so they have to add another acceptable sentence.

The 'useumnida' is the right pronunciation.

"usseumnida" is correct. Look up "Batchim pronunciation" to get a look at how the sounds between the final consonant and the following consonant change. It'll help you get an idea of how many words should sound while reading them.

You've already learned at least one of them. When ㅂ is followed by ㄴ it sounds like ㅁ