Why Is the Major Branch of the K-40 Decay to Ca-40

Why is the major branch of the K-40 decay to Ca-40 not useful in most cases for whole rock dating?

A) It only works 89% of the time.
B) The Ca-40 that is produced by decay is indistinguishable from the naturally most abundant isotope of Ca that may have been present initially.
C) The decay to Ar-40 is faster and has wider geologic applicability.
D) Ca-40 is itself unstable and further decays to more daughter products.