Questions 47 to 56 are based on the following passage.
So many people use the cell phone so frequently every day. But__47__little is certain about the health effects of its use. Manufacturers__48__that cell phones meet government standards for safe radio-frequency radiation emission. but enough studies are beginning to document a possible__49__in rare brain tumors（肿瘤），headaches and behavioral disorders in children to cause concern. So far, the evidence isn’t 50 on whether the use of cell phones__51__to any increased risk of cancer. In a new trial, researchers asked 47 volunteers to__52__in a project to measure glucose（葡萄糖）consumption in the brain by scanning the brain to see how cells use energy. For both 50-minute scans, the volunteers had a cell phone__53__to each ear. During the first scan, the devices were turned off, but for the second scan, the phone on the right ear was__54__on and received a recorded-message call. although the volume was muted（消音）so the noise wouldn’t bias the results. The results of the second scan showed that the 55 of the brain nearest to the device had higher rates of glucose consumption than the rest of the brain. The study shows that cell phones can change brain activity, and__55__a whole new avenue for scientific inquiry, though it doesn’t say anything about whether cell-phone radiation can cause cancer.

50（）