There is a signboard at the corner of the entrance to the mahjong room of the elderly activity center. This signboard is made up of three extra-large dice stacked together, as shown in the figure.

On the top dice, you can see 1, 2 and 3, leaving 4, 5 and 6.

In the middle of a dice, you can see the numbers 4 and 6, so there are also the numbers 1, 2, 3 and 5;

On the bottom of the dice, you can see the numbers 1 and 5, leaving four numbers, namely 2, 3, 4 and 6.

The sum of the invisible points is 4+5+6+1+2+3+5+2+3+4+6 = 41.

So choose C.