Confidence intervals should be used with caution with time-series data because ________.

A) it's difficult to keep track of the time for each sample
B) measuring the variance of time-series data is not possible
C) the mean and/or variance of time-series data are not stationary
D) it's difficult to measure the mean and/or variance of time-series data

Time-series data often have changing mean and/or variance over time, which violates the assumption of independence required for confidence intervals. This means that the confidence interval may not accurately represent the true variability of the data, leading to incorrect inferences. Therefore, caution should be taken when interpreting confidence intervals with time-series data.