by chance （= accidentally， by accident）

偶然 considerate （=thoughtful of the needs ）

体贴的，考虑他人需要的，considerable

相当大的，值得考虑的 consist of

accidental [fortuitous] phenomena;

偶然现象

accident; a chance event;

偶然事件

an occasional cause;

偶然原因

run into an old acquaintance; meet an old friend by chance; come across an old friend;

偶然遇见一个老朋友

an incidental acquaintance;

偶然相识的人

The occurrence of such things is by no means fortuitous.

出现这种问题绝不是偶然的。

I struck upon that antique in a little shop in London.

我在伦敦一家小店里偶然发现那件古物。