The popularity of the cafe is largely due to the boom in cultural and ecological tourism in recent years, combined/ combining with the government's efforts at environmental preservation.

请教老师，combined是题目给出的答案，解释说是作boom的定语，boom和combine之间是被动关系。但是这样的话combine动作的执行者是谁，谁在执行combine A with B这个动作呢？

我的理解应该填combining，因为combine也可以是不及物动词，词典例句：Hydrogen combines with oxygen to form water.且这样也可以解释为the boom和the government's efforts的主动结合，不需要额外的一个动作执行者。

还请老师指点迷津。