The US innovation of Afghanistan was a response to the 9/11/01 attacks on the US. So A.
A historian using the historical thinking skill of examining historiography might want to pay close attention to how the thinking about a certain historical period has changed over time, in light of new evidence.
A trough is a low turning point or a local minimum of a business cycle. The time evolution of many variables of economics exhibit a wave like behaviour with local maxima (peaks) followed by local minima (troughs). A business cycle may be defined as the period between two consecutive peaks.
