问题如下：
Default-free real interest rates tend to be relatively high in countries with high expected economic growth because investors:
选项：
A. increase current borrowing.
B. have high inter-temporal rates of substitution.
C. have high uncertainty about levels of future consumption.
解释：
A is correct.
The average level of default-free real interest rates is positively related to the expected rate of growth of the underlying economy and also to the volatility of economic growth in the economy. During periods of high expected economic growth, investors are less worried about the future and their consumption abilities in the future—that is, their inter-temporal rate of substitution is low, so they borrow more today and save less. Other things being equal, this means that the average level of default-free real interest rates (the reciprocal of the rate of substitution, see Equation 4) should be higher in an economy with high growth, and lower in an economy with lower, more stable growth.
考点：Real default-free interest rates
解析： economic growth增加，投资者的收入会增加，不担心未来没钱，所以储蓄减少，增加消费，所以信贷消费也会增加， current borrowing增加， A正确。current borrowing增加推动利率上涨，所以 default-free real interest rates 较高。
B，结论：经济增速变快，m减小。因为m=MUt+s/MUt, 经济变好，将来收入增加，投资者更有钱了，所以将来消费的爽度减小，MUt+s减小，所以m减小。
C，经济好的时候，将来消费的不确定是比较低的。因为经济好，投资者不担心将来没钱，所以会减少储蓄，增加消费，因此将来消费的不确定性低。
没有在讲义里找到关于current borrowings 增加的内容啊