You can use 잡다 instead of 던지다 Duolingo have conjugated every verb in 읍니다 form actually its polite form the everyday used form is present form아/어/여 + 요. You can listen to Talk To Me In Korean on SPOTIFY. It's a great help
What was your entire sentence? "The child throws the ball." was correct for me. Perhaps you had the multiple choice, where there can be more than one correct answer and all correct answers must be put.