Delhi Transport Corporation and private city buses will be converted to single fuel mode on compressed natural gas by March 31, 2001, the Lok Sabha was told on Monday. The minister of state for environment and forests, Mr Babu Lal Marandi, in a written reply in the Lok Sabha, said all pre-1990 autos and taxis will be replaced with new vehicles on clean fuels by March 31, 2001 in Delhi for control of emission of toxic gases. Buses over eight years old will only ply on CNG or other clean fuels by April 1, 2000, the minister added.