CR doubt question

[shankhamala28]

This question is from Princeton Review:

Measuring the productivity of service workers is a fairly simple matter, mathematically speaking. Simply divide the number of service operations they provide in a day by the amount of time worked in a day to arrive at a numerical representation of their productivity.

Which of the following us NOT an assumption made in the above argument?
(A) The no. of times a service worker accomplishes a task is the only behavior that should be taken into account when calculating productivity.
(B) Tracking the no. of service operations a service employee does in a given time period is an easy task.
(C) All the operations of a given service worker are of equal value, as far as calculating productivity is concerned.
(D) The quality with which an operation is carried out is irrelevant to the calculation of productivity.
(E) The no. of mistakes made in performing operations should not be considered when calculating a worker's productivity.

The answer is given as (B).

I am confused between answers A and B.
This what I thought while solving this question, though I chose the wrong answer (chose A).
If I consider Choice B as an assumption and apply the Negation rule on it, I would get the following:
Tracking the no. of service operations a service employee does in a given timeperiod is not an easy task.
If this is not an easy task, then will be difficult to calculate the productivity as the formula of the productivity is dependent on the no. of service operations carried out by a worker in a day.
Therefore, on negating choice B, the argument seems to fail.
So choice B should be an assumption.

If I consider choice A, it says that the "no. of times a service worker accomplishes a task" is the only behavior that should be taken into account when calculating productivity. But how can this be the only behavior to determine the productivity. I don't understand how does this qualify to be an assumption.

[RonPurewal]

you're missing some important text here (red):

Measuring the productivity of service workers is a fairly simple matter, mathematically speaking. Simply divide the number of service operations they provide in a day by the amount of time worked in a day to arrive at a numerical representation of their productivity.

you have to read the red part literally: the passage is saying that the mathematical calculation of productivity is "simple". this DOES NOT mean that collecting the relevant data is simple; that's not part of the math.

If this is not an easy task, then will be difficult to calculate the productivity as the formula of the productivity is dependent on the no. of service operations carried out by a worker in a day.

... but not mathematically difficult.
the difficulty here would be in the logistics of collecting the data -- not in carrying out the calculations -- so this consideration does nothing to invalidate the argument.

If I consider choice A, it says that the "no. of times a service worker accomplishes a task" is the only behavior that should be taken into account when calculating productivity. But how can this be the only behavior to determine the productivity. I don't understand how does this qualify to be an assumption.

well, just do the "negation" thing again, as you did above.

if you negate this, you get "other things have to be taken into account, too."
if you have that, then this argument is clearly dead, because it doesn't take those other things into account.