How did early models of IBM $360$ solve the problem of running multiple programs concurrently without memory abstraction$?$

Group of answer choices

By dividing memory into $2-$KB blocks and assigning protection keys to each block and trapping attempts to access memory with different protection keys held in special registers.

By swapping programs in and out of memory

By using threads to provide parallelism

By using Least Recently Used $($LRU$)$ page replacement algorithm