As of Date, the Most Significant Challenge to the Positive

As of date, the most significant challenge to the positive empirical results linking trade and growth is the issue brought up by Rodriguez and Rodrik, namely that:

A) there is significant correlation between the variables used in past empirical models.
B) statistical non-stationarity issues have cast a doubt on the validity of past empirical findings.
C) international trade variables capture the influences of other omitted variables, such as institutions.
D) past studies only used fast-growing economies that guaranteed a significant trade-growth relationship.