You can
put this solution on YOUR website! 
The "C" answer is really silly because it goes backward to the given identity.
Notice that "C" is the same as the original given identity.
The "D" answer is wrong because it changes the left side cos²B-sin²B to -1.
That's incorrect. If the left side had been cos²B+sin²B or sin²B+cos²B that
could be changed to 1. Or if it had been -cos²B-sin²B it could have been changed
to -1. The person who thinks "D" is correct is careless with signs.
The "A" answer makes the mistake of removing parentheses. When a parentheses is
preceded by a minus sign or a negative sign, all the terms inside must change
their signs.
The answer "B" is like answer "A" but it doesn't make the mistake that "A" makes.
So it's the correct choice.
Edwin
