Please help me answer this question with explanations

Describe one causal inference challenge that makes it difficult to study the relationship between foreign aid and economic growth. (You might find it helpful to use the concept of "selection" or "threats to identification" in your answer.) For the toolbar, press ALT+F10 (PC) or ALT+FN+F10 (Mac). BIUS Paragraph Arial 10pt . . . V A . .. E = X2 X 2 + ABC