问题
选择题
The scientists were trying to look for all possible _______ as to where this species came from.
A．signals
B．marks
C．clues
D．spots
答案
答案：C
题目分析：句意，科学家们正在努力寻找一切关于人类从哪里来的线索。Signals信号marks痕迹clues线索spots地点，根据题意，故选C。
点评：首先要理解句子意思，同时要知道各个选项的词义，把各个词义放入句中进行比较，不难选出正确答案。