完成下列各题 A
Like everyone else working at the Limerick nuclear-power(核能)plant，Stanley Watras had to pass through the monitors(监测器)before leaving the buildin9．Most of his co-workers．passed through without a problem，but Watras，an engineer，continually set off alarms(警报)．Some days he was found to be carrying six times more radiation(辐射物)than normal． Neither Watras nor his co-workers could understand where he was picking it up． Then one day Watras went through the door at Limerick and turned and walked back through the monitors without ever entering the power block．Yet the machines still said he was carrying radiation．“If I wasn’t picking up radiation at work,there was only one place it could be coming from：my house．” When scientists came to test the Watrases’home in the countryside，they found out what was the matter．The house contained so much radon(氡)that living in it for a year was like being exposed(暴露)to 260，000 chest X-rays．In the year the Watrases had spent there，they had increased their chances of getting lung cancer(癌症)by 13 or 14 percent． The next day the Watrases took down their Christmas tree，put their clothes in some bags and moved into a nearby hotel．“It was terrible，”says Watras． The owner of the Limerick plant took charge of dealing with the Watrases’radon problem as an experiment．Scientists studied every comer of the house．When the ground was dug up，they found that under the house there was a uranium(铀)-beating rock． It was because________ that Stanley Watras was carrying six times more radiation than normal．

A. he worked at a nuclear—power plant
B. there was something wrong with the monitors
C. he lived in a house in the countryside
D. his house was built on a uranium-beating rock