English Grammar - Practice

I’m a native U.S. English speaker and was trying this app for a friend of mine. I went through the “articles” questions and many of the answers were incorrect (“____ unit means a measurement” showed the correct answer as “the,” for example). The “big, great, high, large, and tall” section was also disappointing. For example, one question was “Look at those ______ trees by the lake.” Big, great, large, and tall all work in that blank, but only “tall” is correct according to this app.