Regressing GDP Per Capita on a Measure of Openness to Trade

Regressing GDP per capita on a measure of openness to trade across countries may give biased estimation results because

A) each country measures openness differently
B) without further restrictions, the regression coefficient for openness could be negative
C) openness may capture the effects of omitted variables such as financial reform
D) openness is an index measure, but GDP per capita is measured in monetary units
E) regression is an appropriate econometric methodology only for time series, not for cross-sectional data