You may wonder why anyone would want to weigh a building! Well, engineers 　1　a tunnel or underpass need to make it strong 　2　to support the weight of the ground and any buildings 　3　
　　Building a tunnel under a new building is 　4　problem. The quantity surveyors will know how heavy the building is 　5　it is different when old buildings are 　6　Plans may have been lost, or perhaps they never existed. So surveyors 　7　have to spend weeks or months measuring and 　8　Even then, results might not be accurate.
　　When engineers in Moscow wanted to build a railway tunnel under the 　9　, marble and granite Hotel Moskva, built in the 1930s, they decided to 　10　cosmic rays from space to discover how much weight had to be supported by the tunnel.    
　　Cosmic rays from outer space pour over us at a　11　rate from all directions. Some are 　12　and penetrate right through the earth, just 　13　X-rays pass through our bodies. Weaker ones are stopped by soil and building, so 　14　me as- uring how many cosmic rays penetrate through the soil and buildings above the 　15　tunnel, the weight the tunnel will have to support can be calculated 　16　the Russian engineers dug a tiny pilot tunnel under the hotel and placed cosmic ray 　17　in it. By measuring how many rays had passed through the hotel and the hotel ground above the pilot tunnel, they 　18　out that the Moskva weighs 45,000 tons. This 　19　a ground pressure of 1kilograms 　20　square centimeter, so the railway tunnel will have to be strong enough to take this pressure.______

A.per 

B.every 

C.each 

D.one