A typical (36) language contains an applicative sub-language which approximates the mathematical abstractions of "timeless" functions applied to’ "spaceless" values, where the actual operation sequences and use of storage space during expression evaluation are organized behind the (37) . In this setting, values are data structures of low volume, typically a few computer words or less, which means that an illusion of spacelessness can be realized by having (38) results during expression evaluation stored at the discretion of the language implementation, and effecting parameter (39) and (40) operations through value copying.
39（）
A. imperative
B. mandatory
C. compulsory
D. voluntary