Critical Reasoning

Yorco and Zortech are 2 corporations that employ large numbers of full-time workers who are paid by the hour. Publicly available records indicate that Yorco employs roughly the same number of such hourly wage workers as Zortech does but spends a far highter total sum per year on wages for such workers. Therefore, hourly wages must be higher on average at Yorco than at Zortech, since ____.

A. Zortech spends a higher total sum per year than Yorco does to provide its hourly wage workers with benefits other than wages.
B. the work performed by hourly wage workers at Zortech does not require a significantly higher level of skill than the work performed by hourly wage workers at Yorco does.
C. The proportion of all company employees who are hourly wage workers is significantly greater at Yorco than it is at Zortech.
D. Overtime work, which is paid at a substantially higher rate than work done during the regular work week, is rare at both Yorco and Zortech.
E. the highest hourly wages paid at Yorco are higher than the highest hourly wages paid at Zortech.

Few questions regarding this one:

1. Is this an assumption question type? I am really confused what type of question is this?

2. Also I read this explanation sometime back and saved it with me

Question says there are N employees in Y and Z. Y spends more on
wages than Z.

So, only 2 things are possible -- number of hours that people work
at Y > Z or hourly wage at Y > Z (or both).

We are asked to prove hourly wage at Y > Z which means that
number of hours should be same -- this includes overtime work.

A and B are out of scope. C violates question stem.
E includes samples of highest hourly paid wages -- we are talking
about average here. So, cannot conclude unless we know precisely
how many people we are talking about.

I agree with explanations as given for a b c and e.....But I dont understand how D is the correct option here....can anyone please elaborate this one in more details...

Thanks!