The phrase "ignited the long-smoldering resentments" in paragraph 13 suggests this about the black grassroots organizations: A. Their discontent with apartheid had developed over time and the new laws only made their hatred of it even stronger.
When sentiments are said to be smoldering, it means that they had been existing in a suppressed state. So, the resentments of the people had been existing in a smoldering state.
So, the new laws by the black grassroots organizations fanned the existing flames of resentment in the people.
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