AIDS researchers have e（）(run into) a variety of unexpected problems in their efforts to develop an effective vaccine. （请根据给出的首写字母，填入完整单词的适当形式，其意义与括号中的单词或短语相同）
正确答案：encountered
AIDS researchers have e（）(run into) a variety of unexpected problems in their efforts to develop an effective vaccine. （请根据给出的首写字母，填入完整单词的适当形式，其意义与括号中的单词或短语相同）
正确答案：encountered