Complete the following sentence with a (compound) word derived from the one(s) given in bracket.
All these issues have been（ ）researched into in recent years. (extensive)
【正确答案】：EXTENSIVELY
【题目解析】：
本题考查的是常见形容词的副词。
由researched可知，空缺处应填副词进行修饰，extensive的副词形式是extensively意为“广泛地”。
句意：所有这些问题在近几年都已经被广泛地研究过。