The discovery was made , which we hadn’t expected .
A. on purpose
B. at present
C. in advance
D. by accident
网友回答
A. on purpose
B. at present
C. in advance
(答案→)D. by accident
解析：on purpose故意的；at present现在的，目前的；in advance提前；by accident偶然的。根据which we hadn’t expected，我们没想到会有这个发现，所以应是偶然发现的。选D。