问题如下：
A prolonged period of an official interest rate of zero without an increase in economic growth most likely suggests:
选项：
A.quantitative easing must be limited to be successful.
B.there may be limits to the effectiveness of monetary policy.
C.targeting reserve levels is more important than targeting interest rates.
解释：
B is correct.
A central bank would decrease an official interest rate to stimulate the economy. The setting in which an official interest rate is lowered to zero (the lowest value that could be targeted) without stimulating economic growth suggests that there are limits to monetary policy.
考点：Limitation of Monetary Policy .
当利率水平降低到0时，还无法刺激经济发展（货币政策失效），那么经济就进入了流动性陷阱的状态，所以B选项正确。
经济处于流动性陷阱时，QE政策是最有效的解决手段，所以A选项不正确。
targeting reserve 并不会会影响利率，它并不比 targeting inflation重要， 所以A选项不正确 。
targeting reserve 指的是什么？会有什么影响？
