For simplicity, let 1H be a, 2H be b, 3H be c, 16O be d, 17O be e, and e, 18O be F. 。

A water molecule consists of two hydrogen atoms 1 oxygen atoms. That is to say, you have to choose two from ABC and one from DEF, which makes three altogether. ..

There are six kinds of hydrogen atoms, namely AA, BB, CC, AB, AC and BC.

Then for each choice, it can correspond to the choice of three kinds of oxygen atoms, namely DEF:

Ade, AAE, AAF

BBD、BBE、BBF

………………

Wait ~

1H- 1H- 16O

1H- 1H- 17O

1H- 1H- 18O

1H-2H

1H-2H

1H-2H

1H-3H- 16O

1H-3H- 17O

1H-3H- 18O

2H-2H- 16O

2H-2H- 17O

2H-2H- 18O

2H-3H- 16O

2H-3H- 17O

2H-3H- 18O

3H-3H- 16O

3H-3H- 17O

3H-3H- 18O

Note: I am a sophomore. I'm not in senior three yet. In my senior year, I will focus on combinatorics. At that time, such a question would be very simple. You look like you just graduated from Grade Three or Grade One. Now we don't understand combinatorics, and even if someone lists it, we can't understand its meaning. So now such a problem can only be solved in this way.