Recently，Chongqing Three Gorges University has held an activity named"Cell Phone Ninja"（手机忍者）．They would like their students not to use cell phones for 21days．It is really a challenge （挑战） for most students．
The activity started on April 12．Each student who volunteered to take part 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 appear on the screen of the bracelet．After the last class of each day，the students sent the photos to the school's public WeChat．
More than 800students wished to take part in this test，and 400were chosen．After seven days，only 103students remained．The test was set for 21days because 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 themselves to make the decision．The teachers hoped their students could form better study habits through activities like this one．

71．When did the activity"Cell Phone Ninja"start？
72．Were 400students chosen to take part in this test？
73．Why was the test set for 21days？
74．What was the bracelet used for in this test？ ．