21. No deduction is allowed in computing the taxable value of property transmitted owing to death, for(a) any debt for which there is a right to reimbursement, to the extent that such right may be exercised;
(b) any debt or part of a debt which, considering its nature and the circumstances under which it was created or is claimed, would factitously reduce the estate of such person; or
(c) any obligation assumed in a marriage contract, to the extent that it is not fulfilled at the time of death.