The "main storage" normally means that part of internal storage into which instructions and other data must be loaded for subsequent execution or processing. In large computing systems, the term "main storage" is(71) to "memory". "Buffer" mainly indicates a special-purpose storage or storage area allowing, through(72) storage, the data transfer between two functional units having different transfer characteristics. A buffer storage is used between non-synchronized devices or where one is(73) and the other is parallel or between those having different trans far rates. "(74) (memory)" is also a special-purpose buffer storage, smaller and faster than main storage, used to hold a copy of instructions and data obtained from main storage and likely to be needed next by the processor. "Register" is commonly composed of a part of internal storage having a specified storage(75) and usually intended for a specific purpose.

A. suitable
B. optional
C. choice
D. preferred