The purpose of the 1M resistor is to protect you from being wired to a solid earth, in the event of another piece of equipment having a live to chassis fault.
Point A will protect you, if you are grounded nowhere else.
Point C would be good as well, as it protects you even if you are touching the (fairly conductive) mat.
The laptop ought to be isolated by its PSU, but do you trust those cheap small hot running things? Put another one at B, they're only a penny each.
Basically one in every lead you own or use will offer you the best protection against shock, however you configure the leads, and will not degrade the protection against ESD.