1.（2019全国I卷）Modem methods ______ tracking polar bear populations have been employed only since the mid-1980s, and are expensive to perform consistently over a large area.
【答案】of/for
【解析】考查介词。此处tracking polar bear populations作Modern methods的定语，用of 连接，“methods of doing sth.”,意为“…的方法”，构成固定结构。或者意为“对于跟踪北极熊的方法”用for。故填of/for。
2. （2019全国III卷）We were first greeted with the barking by a pack ______ dogs，seven to be exact.
【答案】of
【解析】考查介词。“a pack of”意为“一群”。故填of。
3.（2019浙江卷）The answer ______ this question is not clear.
【答案】to
【解析】考查介词。句意：这个问题的答案不清楚。the key/answer to ...这是固定搭配，“……的答案是……”。故填to。
