(a)  Whenever a trustee or a tenant is authorized by the terms of the transaction by which the principal was established, or by law to use any part of the principal in the continuance of a business which the original owner of the property comprising the principal had been carrying on, the net profits of such business attributable to such principal shall be deemed income.

(b)  Where such business consists of buying and selling property, the net profits for any period shall be ascertained by deducting from the gross returns during and the inventory value of the property at the end of such period, the expenses during and the inventory value of the property at the beginning of such period.

(c)  Where such business does not consist of buying and selling property, the net income shall be computed in accordance with the customary practice of such business, but not in such way as to decrease the principal.

(d)  Any increase in the value of the principal used in such business shall be deemed principal, and all losses in any one calendar year or fiscal year if it differs from calendar year, after the income from such business for that year has been exhausted, shall fall upon principal.