Which of the following is true?

(a) Gap device a method to accommodate future subjects, is used at every level of arrays in almost all classifications. 

(b) Sectorising Digits are devices to accommodate a subject at a proper place in an array even if no vacant place is there.

(c) U, W, and Y have been postulated as Empty digits.

(d). Digits T, V, and X are set aside as Empty-Emptying Digits as these empty a preceding digit of its meaning but allow it to retain its ordinal value.