How do I make my fill-in-the-blank exercise more obvious?

I developed an app for learning Chinese that requires the user to do fill-in-the-blank exercises. A word in the sentence will be blank, and the user has to select which options is correct. It looks like this:

Users seem to be able to understand what they are supposed to do there. However, some users are confused by exercises like this one:

They don't notice that the character for "two" is already present and that the blank should be filled with "twenty". Instead, they look for a button that has "twenty two" and are confused when they can't find it.

Is there a better way to present the exercise to make it more clear that they need to fill in the blank, rather than find the translation of "twenty two"?