Applications put computers to practical business (67) , but below the (68) it’s the heart of an operating system-the kernel-that provides the technical wizardry to juggle multiple program, connect to networks and store (69) .
A traditional kernel provides all the functions for applications. The kernel (70) memory, I/0 devices and parcels out processor time.
The kernel also supports security and fault (71) , which is the ability to recover automatically when parts of the system fail.

A．used

B．use

C．apply

D．applied