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 . She insisted on to participate (a) / in the discussion (b) / despite lacking sufficient preparation (c) / on the subject (d). / No error (e)