C, presumably to avoid the warning messages about entering a formula when a number is expected. So they have intentionally disabled the type warnings, and allowed the kinds of unusual answers like you have given to be marked correct.
Not only that, they have set things up so that wrong warning messages will now be issued. For example, if you typed
1,2into the answer, you will get the message "Your answer isn't a formula that returns a number (it looks like a list of numbers)", which incorrectly suggests to the students that they should be tying a formula rather than a number.
Such efforts to bypass the standard warnings are ill-advised and usually cause worse problems than the ones they were trying to avoid. It is rarely a good idea to intentionally use the wrong type of answer from what the students are supposed to type.
Had they used MathObject directly, they could have disabled the type warnings with a flag, and left the answer a real number, so that a formula like your
x/xwould have been marked incorrect, but without any warnings about the wrong type of answer.
I agree with you, this is poor coding. Unfortunately, it is not restricted to the Rogawski problems.
PS, thanks for the kind words about the Union problems.