From a historical computing perspective, why were central (host-based or time-shared) systems widely used before distributed personal computing became common?

Introduction / Context:
Early computing emphasised centralised architectures such as mainframes and minicomputers. Users accessed shared resources via terminals because the economics and technology of memory and processing favoured consolidation. Understanding these drivers clarifies why the industry later shifted as semiconductor memory matured and costs dropped.

Given Data / Assumptions:

• Era considered: pre–ubiquitous semiconductor RAM and before inexpensive microprocessors and PCs.
• Memory availability, cost, and power draw were key constraints.
• Centralised systems amortised high fixed costs across many users.

Concept / Approach:
Magnetic core memory and early semiconductor memory were expensive and power-hungry. Centralising compute and memory let many users time-share a powerful system, improving utilisation and making costs manageable. As DRAM densities increased and prices fell, decentralised personal computing became economically viable, reversing many of the earlier constraints.

Step-by-Step Solution:

Verification / Alternative check:
Historical sources note core memory’s high cost and power, with time-sharing systems maximising utilisation.

Why Other Options Are Wrong:

Common Pitfalls:
Projecting today’s cheap memory costs backwards; underestimating power and cooling limits in older data centres.

Final Answer:
All of the above.