Some scientists have questioned the theory ________ CO₂ is causing dangerous global warming．

A．which

B．what

C．that

D．why

They studied what ________ since 2007 and found Australian electricity prices were among the highest in the developed world．

A．has happened

B．happened

C．had happened

D．has been happening

It's not how much money you make or who your daddy is ________ makes you the most attractive person in the world but the way you do some simple things．

A．that

B．which

C．what

D．who

A study has found that 80％ of the people who ________ to be constantly distracted ________ more “working memory”．

A．appears; have

B．appear; has

C．appears; has

D．appear; have

In May 2011, Americans spent 53 billion minutes on Facebook, ________ it the most popular U．S．website．

A．making

B．made

C．to make

D．having made

A Colorado woman said that her iPhone 4 ________ fire while charging overnight．

A．had caught

B．has caught fire

C．was catching

D．caught

More and more old people are skiing but it has ________ to do with snow and slopes：this type of skiing stands for Spending the Kids Inheritance．

A．something

B．nothing

C．everything

D．anything

David is so curious that no one can ________ from finding out what is going on．

A．hold him back

B．hold him up

C．hold to him

D．hold him down

What the salesman said was so persuasive that the old man was completely ________．

A．taken in

B．given in

C．brought in

D．turned in

There seemed to be no human beings in that village, and the only ________ of life was a boat tied to a tree．

A．symbol

B．mark

C．signal

D．sign