---they have found someone in trouble,they decide on suitable ways to help them.

B吧
A是虚拟，虚拟条件句时态要比结果句时态过去一个等级
如果A后面应该是will decide
而且have found暗示是once

once强调他们会立刻想办法帮助他们
if只说他们会帮助他们，没有强调什么时间会去帮助他们

___Since____they have found someone in trouble,they decide on suitable ways to help them.
A If B Once C Because D Since
【解析】if从句一般都是“主将从现”，因为条件状语都是假设的情况，此题也不是一楼说的虚拟，如果是的话应该是 if had done , would have done/if did, would do

b 一旦他们发现别人有麻烦，就会采取适当的方式帮助他们
用if的话，就是if they find