Most computer systems are (71) to two different groups of attacks：insider attacks and outsider attacks．A system that is known to be (72) to an outsider attack by preventing (73) from outside can still be vulnerable to the insider attacks accomplished by abusive usage of (74) users．Detecting such abusive usage as well as attacks by outsides not only provides information on damage assessment,but also helps to prevent future attacks．These attacks are usually (75) by tools referred to as Intrusion Detection Systems．

（72）处填（）。

A．reliable

B．secure

C．indestructible

D．steady

既是自然界中最多的矿物，也是非金属矿物的是（）

A.磁铁矿

B.赤铁矿

C.石英

D.方铅矿