So here's the solution.

We start with our basic orifice equation again.

And in this case, the upstream diameter here,

say the diameter at this point, we can put equal to infinity, or a very large number,

in which case this term, 1 minus d over d to the 4th, drops out.

So the equation then is given here, or

this, substituting for the pressure difference for the height difference where

h is the height difference between the two water levels here and

using our equation for the orifice coefficient cv times cc etc.

Which is, if D is very large, this term drops out.

So it's essentially equal to the velocity coefficient times the contraction

coefficient or 0.98 x 0.83, which is 0.81.

And now we can substitute into the equation, and

the answer is 7.97 times 10 to the minus 4 meters cubic meters per second

multiply that by 86,400, which is the number of seconds in a day.

We get our answer 68.9 cubic meters per day.

So the closest answer is A.