\textbf{180.} Two identical metal spheres A and B with radii 5 cm having electric charges $q_A = 20\,\mu\text{C}$ and $q_B = -4\,\mu\text{C}$ are brought into contact and then separated. How many microcoulombs does the surface charge density of sphere A decrease per square meter? $(\pi = 3)$
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(1) $150$ \quad (2) $300$ \quad (3) $400$ \quad (4) $800$
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