I took the test a couple years ago so I could custom spray with my own rig.  I read the core A book through and through for about a week and never got the core C book, I just winged that test.  I passed both with flying colors.  I would not take the test as a joke.  The core C test has quite a few repeat questions from the core A test.  If you do not read the core A book well, I feel the test would be very tough.  If you have been farming for awhile or have an agronomy degree the core C is easy.  One thing I noticed about the core A test was that none of the practice questions at the end of each chapter were in the test, so make sure you read well, don't just study the questions. 

Good Luck