div covering problem_html/css_WEB-ITnose

There are two DIVs "A" and "B". Among them, "A" DIV is nested in another DIV "C". A and B are positioned relatively, and C is positioned absolutely. Question: Without changing the positioning, how can I make part of C cover B
This is probably what it looks like. z-index has been used but it has no effect. A: Z-INDEX=99 B: Z-INDEX=44 is useless. Ask God for a solution.

Reply to discussion (solution)

C Why use absolute positioning? How to set it up? Set z-index to C and try again

Also set position: relative; and z-index: ;

The problem has been solved. It's all the fault of relative positioning.