New Delhi, Sept 08 (ANI): The Supreme Court on Monday said the allegations against CBI chief Ranjit Sinha in the 2G and coal allocation cases are serious and asked him to respond to them. Referring to the charges raised by activist and lawyer Prashant Bhushan that many of the main accused in the two high profile cases were frequent visitors at the CBI chief's residence, the court asked him to file a counter-affidavit within a week.