You may have seen this tank filling puzzle that's gone viral. But have you wondered what happens at a faster flow rate? Someone has tried it out, with a 3D printer. First though, let's look at the original puzzle.
Here it is.
Look closely, as it says.

Most people answer “G”.
If that's your answer too, take another close look. Many of the pipes are blocked, The line that blocks off D from C is not a mistake.
To find the real answer - well first, it looks like it's just a drip at a low flow rate from the drawing. So let's assume that..
From A to B to C is straightforward. None of them can fill before the next one.