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3) C

4) I think its B not sure.

Explanation:

3) Standing waves are formed by the superposition of two travelling waves of the same frequency (with the same polarisation and the same amplitude) travelling in opposite directions. This is usually achieved by using a travelling wave and its reflection, which will ensure that the frequency is exactly the same.

4) Interference is a phenomenon in which two waves superpose to form a resultant wave of greater, lower, or the same amplitude.

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the answer is neither ball has a charge.

because if they were both positively charged, they will repel each other.

if they were positively and negatively charged, they will seek.