Which of the following observations would most contradict the weak-form efficiency of the stock market?

A. Average market raw return is positive

B. Insiders can earn an abnormal return by trading on their inside information

C. Momentum strategy that buys past 12-month winners outperform the market every month even after controlling for the level of risk

D. Over 25% of mutual fund managers outperform the market

Please explain why each answer either contradicts or affirms weak-form efficiency.