solve, solved, solving; all, either, whole
The past is filled with incredible mysteries. The clues to ______ them are ______ around, hidden in plain sight.
【正确答案】：solve; all
【题目解析】：To后面跟动词原形是不定式作定语修饰the clues所以用solve；所有的线索到处都是，all around指到处都是，either指两者之一，whole不能修饰around。
