(a)the [Assessing Officer] [Substituted by Act 26 of 1988, section 72, for 'Income-tax Officer' (w.e.f. 1-4-1988).] has reason to believe that by reason of omission or failure on the part of the assessee to make a return under section 8 for any assessment year or to disclose wholly and truly all material facts necessary for his assessment for any assessment year, chargeable expenditure for that year has escaped assessment or has been under-assessed, or
