What are these - underside of engine (5.3L)?

[2006Tahoe2WD]

On the drivers side, back, lower part of the engine there are these items I'm not sure of:

A - some sort of plug (water jacket drain or ? oil galley drain)
B - same question as A

C - small manifold/fitting that has nothing connected to it. If I feel on top there is a hole - can't tell if it is threaded or not. From the location I would guess it is associated with the oil system. Maybe a point to hook in an oil cooler but that option was not attached or someone removed it.

Thanks in advance for your expert knowledge. Vehicle = 2006 Tahoe, 5.3L, 2WD

 

[2006Tahoe2WD]

OK, OK - you are going to make me look it up.
After pouring over the service manual I finally tracked down the figures showing these items. Still I need to confirm other coolant drain plugs on the block. I think there is another one on the left and at least one on the right side of the engine. Apparently I don't have the factory oil cooler option - the "C" item is a block off. Attached is an updated picture. What is the size of the Allan tool needed to take out the coolant plug? I think I saw it is 17mm. That's a nice handy size.