You really hear a "b" sound in it: https://www.duolingo.com/dictionary/Greek/%CF%84%CF%81%CF%89%CF%82/bbad7442ede86d7268f993eb22c8b212 ?? I just hear "tros" with a rolling, Greek r.
It says "this" because the Greek sentences says "Εσύ τρως αυτό το σάντουιτς." and "αυτό" means "this". If it had said simply "Εσύ τρως το σάντουιτς." then it would be "the sandwich."
TIPS TO HELP YOU LEARN
1 Use the Drop-Down hints to help you translate. Pass your cursor over a word and a list of translations will appear. **Always choose the top word/phrase.
This will assure that you always have the right translation. It's the secret to success. Do not hesitate to use these as often as you need to.
2 Read the Tips & notes, on the first page of each lesson you’ll see TIPS. Click on that.
3 Always read the comments before posting. Check the heading on the page to see the sentence and its translation. Click on any "blue words* for more definitions and a list of other sentences using those words.
4 If your translation is rejected you should carefully compare what you wrote with the answer given. If you do not see a mistake use the Report options at the foot of the exercise page to Report issues such as My answer should be accepted.
See here for how to Report a problem.