Complete the passage according to the first letters or with the proper forms of the words given．

Recently，Chongqing Three Gorges University has （71） 　  　（hold） an activity named"Cell Phone Ninja"（手机忍者）．They would like their students not（72） 　    　（use） cell phones for 21 days．It is a challenge（挑战）（73）f 　  most students．

The activity started on April 12．（74）E 　  　of the students who volunteered to join in this activity got a bracelet （手环）．If the students did not use their cell phones，there would be a

photo on the screen of the bracelet to show that．If they used their cell phones，a different photo would （75）a 　    　on the screen of the bracelet．After the last class of every day，the students sent these （76） 　   （photo） to the school's public WeChat．

More than 800students wished to join in this test，and 400 were chosen．After seven days，only 103students were left．The test lasted for 21 days （77）b 　   　some scientists believe that 21days is long enough to help form（形成） a habit．

According to the teachers in the university，the activity was not a competition．It depended

on students （78） 　     　（they） to make the （79 ） 　   （decide）．The teachers hoped their students could form better study habits through activities （80）1 　    　this one．