Ray enjoys his job,although the opportunity to get into computing___quite by accident.

应该选A came across

come across的意思是:
来到, 偶遇, <口>给人印象深刻
不期而遇,交付
To meet or find by chance:
偶然遇到或找到：
came across my old college roommate in town today.
今天在镇上遇到我大学的老同学

题目可以翻译为:Ray喜欢他的工作,虽然进入行业的机会是很偶然地得到的

所以题中用come across最佳

附:另外三个短语的意思,这样你就知道它们不恰当了

come over
To change sides, as in a controversy.
改变立场，例如在争论中
To pay a casual visit.
偶然拜访

come about
To take place; happen.
出现；发生
To turn around.
转向
Nautical To change tack.
【航海】 改变航向

come in
To arrive:
到达：
Fall clothes will be coming in soon.
秋装即将上市
To become available for use:
成为可用：
New weather information just came in.
新气象信息刚被采用
To start producing. Used of an oil well.
开始生产。用于油井
To arrive among those who finish a contest or race:
比赛名次：在完成比赛的那些人中达到：
came in fifth.
第五名
To perform or function in a particular way:<