I am taking a Spanish class on coursera. I can't get the following question right, any idea why? I feel my answer is a legit sentence.

I am taking a Spanish class on Coursera. I can't get the following question right, any idea why? I feel my answer is a legit sentence.

Given the pattern:

Ellos / acabar de / llegar / casa

I am writing:

Ellos acaban de llegar a la casa.

However, it says it is not correct: