问题如下：
1.Which of Busse’s conclusions regarding the exchange rate time series is consistent with both the properties of a covariance-stationary time series and the properties of a random walk?
选项：
A. Conclusion 1
B. Conclusion 2
C. Conclusion 3
解释：
C is correct. A random walk can be described by the equation xt = b0 + b1xt–1+ εt, where b0 = 0 and b1 = 1. So b0 = 0 is a characteristic of a random walk time series. A covariance-stationary series must satisfy the following three requirements:
1. The expected value of the time series must be constant and finite in all periods.
2. The variance of the time series must be constant and finite in all periods.
3. The covariance of the time series with itself for a fixed number of periods in the past or future must be constant and finite in all periods.
b0 = 0 does not violate any of these three requirements and is thus consistent with the properties of a covariance-stationary time series.
看了前面人问的问题，有一点想确认下，random walk是不是包含random walk with a drift？如果不是，那random walk和simple random walk就是一回事？
