4. (a) What do you understand by the "source theory" in computing the liability of a taxpayer? (b) Explain, illustrate and distinguish between "wholly and exclusively for the purposes of the trade" and "wholly and exclusively incurred in acquiring the income". 4. (a) What do you understand by the "source theory" in computing the liability of a taxpayer? (b) Explain, illustrate and distinguish between "wholly and exclusively for the purposes of the trade" and "wholly and exclusively incurred in acquiring the income