Correct solutions have to be manually entered. So, the reason why it was "marked wrong" is because it simply hasn't been added yet. To request to have a sentence added, click on the "report" button after answering, and then select "My answer should be accepted".
@Cici, not true. "너무" can be translated as "too" in this sentence. The "extra word" you are talking about here is "a", but it's not incorrect - Korean doesn't have a word for "a", so in most cases it's optional to translate the sentence with the word "a" or not. In this case, it can be translated with "a" and would still be correct.