参考答案

正确答案：their request has been fulfilled
their request has been fulfilled 解析：由题干中的when可知，本空应填一句子成分完整的从句。根据题干中的Ushahidi和senders定位到末段首句。题干中is not clear of the effect of 4636是对文中has no way of knowing how successful it has been的同义转述。senders usually do not give a feedback是对文中people generally don't send messages to say的同义转述，故其后的their request has been fulfilled即为答案出处。