(21) put computers to practical business use, but below the surface it is the heart of an (22) - the kernel - that provides the technical wizardry to juggle multiple programs, connect to networks and store (23) .
A traditional kernel provides all the functions for applications. The kernel (24) memory, I/O devices and parcels out processor time.
The kernel also supports (25) and fault tolerance, which is the ability to recover automatically when parts of the system fail.

A．authentication

B．auto - correction

C．security

D．problem

已知集合U={1，2，3，4，5}，A={1，2，3}，B={2，5}，则A∩C_UB=（　　）

A．{2}

B．{2，3}

C．{3}

D．{1，3}