In node red how to use drop down node for getting multiple values result store

General
1

I wanted 3 drop down list, based on 1st drop down selection 2nd drop down list options should come, and based on 2nd 3rd drop down list should come. The payload is

[{
		"controller": "S000",
		"sensorConf": [{
				"sensor": "s"
			},
			{
				"sensor": "s2"
			}
		]
	},
	{
		"controller": "S001",
		"sensorConf": [{
				"sensor": "s3"
			},
			{
				"sensor": "s4"
			}
		]
	}
]

So after calling API I will get below response, So I placed one drop down node for controller id seletion(ex c1), after that I wanted to choose from array of sensorConf sensor parameter( ex s3).

test1
test1.PNG806×100 6.76 KB

In the above flow how to save the selection result for each request by user. As I have to request again API to fetch the sensorConf deatils(sensor).

Which variable I have to use context variable,Flow variable , Global variable . If I use global then it is global not based on user selection.