设 f ( x ) =log a x ,则( )成立.
A.
f (x) ·f (y) = f (x+y) 应该是 log a x·y = log a x + log a y .
B.
f (x)+f (y) = f (x+y) 应该是 log a x·y = log a x + log a y .
C.
f (x·y) = f (x) ·f (y) 应该是 log a x·y = log a x + log a y .
D.
f (x·y) = f (x)+f (y) 由对数的性质得 log a x·y = log a x + log a y .