Consider the following important sources of tax revenue for the Central Government :
I. Corporation tax
II. Taxes on income other than corporation tax
III. Customs
IV. Union excise duties
Which one of the following is the correct descending order in terms of gross tax revenue?
1. I-II-IV-III
2. I-II-III-IV
3. III-I-II-IV
4. II-III-I-IV