Hello.
The box to open would be: B
Because we've shown that the two statements are both true, we now know that box A has a deadly snake in it (due to the statement on box B). So the box to open is box B since this is the only box with 1 million dollars in it.
the begining says one of the boxes contains 1 mill. so obviously the writing on box A has to be true, otherwise there would be no money in either boxes. so the writing on box B is true so we know there is a snake in box A.
Have a nice day