It is a job ___ you are doing something serious but interesting 你可能会先考虑that,因为it is that可能是强调句.但是这里如果把it is that去掉就成you are doing something serious but interesting a job,显然不成立,所以不是强调句.而是定于从句,准确的应该是you are doing something serious but interesting（ in） a job ,需要介词,所以是where 也可以是in which