Special vacuum _______ are needed when cleaning laser printers

The accompanying figure shows ____________________ view.

Fill in the blank(s) with the appropriate word(s).

In the 1970s and 1980s, what was an early roadblock to concurrent programming?

a) Early operating systems did not allow several programs to execute concurrently. b) Early computers were too slow to permit concurrency. c) Early programming languages did not include primitives enabling programmers to write concurrent programs. d) all of the above