根据Ⅷ：C的活性可将血友病A分为四型，中型血友病Ⅷ：C的活性为（）

A.≤1％

B.2％～5％

C.6％～8％

D.9％～12％

E.13％～16％

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