设X与Y为相互独立的随机变量，且Vat（X)=4，Var（Y)=9，则随机变量Z=2X－y的标准差为（)。A.1

听音频，回答下列问题：

A.Peaceful.

B.Considerate.

C.Generous.

D.Cooperative.

听音频：{MP3:/imgcache/attached/media/20140429/20140429163517_1876.mp3} 回答下列各题

A.Because he prefers something hot.

B.Because he likes tea better.

C.Because he needs some coffee.

D.Because he has to drive home.

听音频，回答下列问题：

A.Time spent exercising.

B.Time spent working.

C.Time spent on leisure activities.

D.Time spent with friends and family.

听音频，回答下列问题：

A.To look for a job as a salesperson.

B.To have a talk with Miss Thompson.

C.To place an order for some products.

D.To complain about a faulty appliance.

听音频：{MP3:/imgcache/attached/media/20140522/20140522113252_8493.mp3} 回答下列各题：

A.Go to a place he has visited.

B.Make her own arrangements.

C.Consult a travel agent.

D.Join in a package tour.

听音频，回答下列问题：

A.She had a desire to help others.

B.She wanted to find out more about it.

C.She needed some overseas experience.

D.She was interested in farming.

听音频，回答下列问题：

A.They have destroyed several small towns.

B.They will soon spread to San Francisco.

C.They have injured many residents.

D.They are burning out of control.

听音频，回答下列问题：

A.The car driver was trying to avoid hitting a rabbit.

B.The car driver was partly responsible for the accident,

C.McLaughlin was talking to his manager while driving.

D.McLaughlins carelessness resulted in the collision.

听音频，回答下列问题：

A.Bring him up to date on the current situation in Milan.

B.Inform. him of the arrangements for his trip in Italy.

C.Fetch the documents signed by Mr Gartner,

D.Accompany Mr Gartner to the Linate airport.

听音频，回答下列问题：

A.She is both a popular and highly respected author,

B.She is the most love African novelist of a J1 times.

C.She is the most influential author since the1930s.

D.She is the first writer to focus on the fate of slaves.