Great question! Being a learning app , Sololearn should prevent this kind of guessing. Not only in challenges. It is much too easy to get through courses by guesing ones way through lessons. This lowers the value of certificates earned, and discourages real thorough learning
I have also noticed that the answer can be partially inferred from the available space to type it into. Does the width of a font change the available space? If not then, perhaps, a variable-width text field can accomplish this.