Dogs wag(摇摆) their tails in different directions depending on whether they are excited and wanting to move forward or threatened and thinking of moving back, a study has found.
Researchers in Italy examined the tail wagging behaviour of 30 dogs, catching their responses to a range of stimuli(刺激物) with video cameras. To conduct the study they chose 15 male dogs and 15 female ones aged between one and six years. The dogs were all family pets whose owners had allowed them to take part in the experiment at Ban University. The dogs were placed in a large wooden box with an opening at the front to allow for them to view various stimuli. They were tested one at a time.       
The researchers led by Professor Giorgio Vallortigara of the University of Trieste found that when the dogs were shown their owners—a positive experience—their tails wagged energetically to the right side. When they were shown an unfamiliar human they wagged to the right, but with somewhat less enthusiasm. The appearance of a cat again caused a right-hand side wag, although with less intensity again. The appearance of a large unfamiliar dog, similar to a German shepherd, changed the direction of tail wagging to the left. Researchers supposed the dog was thinking of moving back. When the dogs were not shown any stimuli they tended to wag their tails to the left, suggesting they preferred company. While the changes in the tail wagging were not easily noticed without the aid of video, it was thought that the findings could help people judge the mood of dogs. Computer and video systems, for example, could be used by professional dog trainers to determine the mood of dogs that they were required to approach.
.The video cameras were used to catch the dogs’ responses because ___________ .   

A．it was easier to catch the dogs’response changes in the tail wagging

B．the dogs were put in the wooden boxes and tested one at a time.

C．they enabled the dogs’owners to know about their dogs’habit

D．the dogs wagged their tails in different directions when they were in different moods

The underlined word “intensity” in the passage means        .

A．surprise

B．worry

C．excitement

D．interest

When there are no stimuli, a dog will ___________         .

A．wag to the left	B．wag to the right
C．not wag at all	D．wag to the left and then to the right

The underlined word “they” refers to _____.

A．the dogs

B．the trainers

C．the systems

D．the rescarchers

The purpose of doing the experiment is _____________ .

A．to train dogs for their owners	B．to help people judge the mood of dogs
C．to help dogs find company	D．to help people choose their pet dogs