问题如下：
The current inflation rate of Country X is 1.5%, while its central bank believes the optimal inflation rate is 2%. In order to reach the target, central bank of Country X should:
选项：
A.sell securities in the open market.
B.increase the percentage of deposits that banks must retain as reserves.
C.reduce the repo rate.
解释：
C is correct.
In order to increase the inflation rate to the target level, central bank can buy securities in the open market, decrease reserve requirements and reduce the policy rate, such as repo rate.
考点：Tools of Monetary Policy.
解析：为了将通货膨胀率提高到目标水平（1.5%到2%），央行可以使用在公开市场上购买证券，降低准备金率，降低回购利率等政策利率等方法，因此只有C选项入选。
policy rate是啥概念？除了包括repo rate还有？
