【6.Mostenvironmentproblemsexistbecauseadequatemeasuresforpreventingthem(A)takeninthepast.A.werenotB.hadnotbeen8.Theprofessor,(A)asasplendidspeaker,waswarmlyreceiverbythestuclents.A.knownB.tobeknownC.havingknown9.】
6.Mostenvironmentproblemsexistbecauseadequatemeasuresforpreventingthem(A)takeninthepast.
A.werenotB.hadnotbeen
8.Theprofessor,(A)asasplendidspeaker,waswarmlyreceiverbythestuclents.
A.knownB.tobeknownC.havingknown
9.Heissaid(C)forfiveyears.
A.tohavediedB.todie
C.tohavebeendeadC.dead
10.Mostofpeople(A)tothepartywerefamousscientists.
A.invitedB.beinginvited
就是不知道为什么不选答案之外的选项?