Barack Obama has taken personal blame ____ the security failures which led ____ the attempt to

blow up a plane on Chrismas Day.[ ]

A. of; to          

B. for; to      

C. on; on            

D. in; on

某烷烃主链为4个碳原子的同分异构体有2种．则与这种烷烃具有相同的碳原子数、主链上也为4个碳原子的单烯烃共有同分异构体（　　）

A．2种

B．3种

C．4种

D．5种