PERTH—Search crews will send a submarine(潜航器)deep into the Indian Ocean on Monday for the first time to try to determine whether signals detected by sound-locating equipment are from the  missing Malaysian plane's black boxes, the Australian head of the search said.
Angus Houst on said the crew on board the Ocean Shield  will send the underwater vehicle sometime Monday evening. The Bluefin 21 submarine can create a sonar(声呐)map of the area to chartany wreckage(残骸)on the seafloor.
The move comes after crews picked up a series of underwater sounds over the past two weeks that were consistent with an aircraft's black boxes.
“We haven’t had a single detection in six days, and l guess it’s time to go under water,” said Houston.
“Analysis of the four signals has reduced the search area on the ocean floor. The experts have therefore determined that the Australian Defense Vessel Ocean Shield will stop searching with the Towed Pinger Locator(拖曳声波定位仪)later today and use the underwater vehicle Bluefin 21 as soon as possible," he said at a news conference in Perth.
But Houston warned the switch to the submarine will not automatically "result in the detection of the aircraft wreckage. It may not.”
He said the submarine will take 24 hours to do each task, including two hours to dive, 16 hours to search the bottom, then two more hours hack up and four hours to download data.
Recovering the plane's flight data and cockpit(飞行员座舱)voice recorders is necessary For investigators to try to figure out what happened to Flight 370, which disappeared on March 8. 
Houston said the search using the submarine will be a slaw and painstaking process.
The submarine takes six times longer to cover the same area as the pinger locator,and will need about six weeks to two months to examine the present underwater zone. The signals are also coming from 4,500 meters (15,000 feet) below the surface, which is the deepest the submarine can dive.
The submarine will be sent deep into the Indian Ocean on Monday to _____ .

A．create a sonar trap of the area

B．pick up underwater sounds

C．see if the signals detected are from the target black boxes

D．reduce the search area on the ocean floor

According to the passage, the submarine_______ .

A．will download data of the black boxes

B．can find the aircraft wreckage within 24 hours

C．will recover the plane's flight data and voice recorder

D．can dive to a depth of no more than 4,500 meters

The underlined sentences in the passage indicate that______.

A．the switch of the submarine may not function properly

B．the Ocean Shield will switch to submarine when it is necessary

C．the submarine will detect the wreckage of the plane automatically

D．the detection of the wreckage can't be guaranteed by using the submarine

The search will be a slow and painstaking process, mainly because _____ .

A．the underwater sounds were consistent with an aircraft's black boxes

B．the submarine travels slowly and the ocean is deep

C．they haven't had a single detection in six days

D．Ocean Shield stops searching wine the towed pinger locator