If one wants a comma inside an expresssion that shall be
visible in the answer, then one has to escape it ` 
,'.
The same holds for the other special characters like `(', `)' etc.
Here is a short list of supported special characters. Some require
a double backslash.
In principal, any non-alphanumerical character like '(',')','{','}', '[', ']' can be referred to by the backslash operator. Note that the vanilla versions of these charcters are used to denote expressions in answer formats. Hence, we need to 'escape' them by the backslash if they shall appear in the answer.