What Is the Main Difference Between WHMIS Hazards Represented by a Circle

What is the main difference between WHMIS hazards represented by a circle containing a large T with a dot on the end from WHMIS hazards represented by the skull and crossbones?

A) They involve different means of exposure.
B) They are different WHMIS classes of material.
C) They produce radiation injury rather than corrosive injury.
D) They need repeated versus single exposure for injury/illness.