Correct Answer (Detailed Explanation is Below)
A.
2-Chlorobenzylidene malononitrile
Detailed Explanation
CS, CN and lachrymatory agents are associated with tear gas. CS is o-chlorobenzalmalononitrile and CN historically refers to chloroacetophenone. These substances cause irritation and tearing rather than functioning as ordinary atmospheric gases. For competitive exams, remember the terms
CS gas, CN gas, tear gas and lachrymator.