Hello. I am looking to install a new smart thermostat which will need a C wire. (First photo) When I took the old thermostat off the wall i found only 4 wires plugged in, there was a slot labeled C but there were no wires plugged in it. There was a separate wire alone not plugged in to any of the slots that is part of the wire bundle coming out of the wall. I went into my attic where the AC unit is located (sorry if I'm getting the lingo wrong) and opened up the panel to show the wire connections on this end (see other photo of panel). I see that the same wires are plugged into their ends here, and again there is a slot labeled C. At this end it looks like the blue wire that was free at the thermostat end is also free here. My question is, can I simply plug that free blue wire into the slot labeled C and plug it into the new smart thermostat on the C connection? Is it that simple? I greatly appreciate the help.