One Reason the Flynn Effect Is Difficult to Explain Is

One reason the Flynn effect is difficult to explain is because

A) it has been so widespread we cannot compare where it occurs to where it has not.
B) the data have been so inconsistent, it is hard to find any patterns in the data.
C) it has only been documented in non-English speaking countries, and there have been difficulties in translation.
D) the effect no longer occurs, and the historical data do not explain why the effect used to take place.