Believe it or not, weather forecasts seven days in advance are, ________, wrong half the time, according to a US survey．
A.at firstB.in briefC.on averageD.by chance在线课程C
语境中考查固定习语的运用。句意：信不信由你，根据美国的一项调查，提前七天的气象预报平均半数时间有误。