Click on the correct answer. It must be grammatically correct (correct English), but it must also be true.
The following sentence, for example, is a true fact, and the English is correct.

Sea water doesn't freeze when the temperature drops to 0 degrees celsius.
(It freezes at a lower temperature.)

The timer gives you 15 seconds to find the answer. It notes your errors but it only shows you the correct answers at the end.

If you don't use the timer and make errors, you see the correct answers immediately.