Each of the following sentences has been divided into five parts (a), (b), (c), (d) and (e).  One of these parts may contain a grammatical or idiomatic error. Identify the part that contains the error. If there is no error, choose option (

• E . The company celebrated its five hundredth (a) / employee joining the organisation (b) / by organising a special ceremony (c) / at the headquarters (d). / No error (e)