The river level was rising drastically after that heavy rain.

The correct answer is C. The river level was rising greatly after that heavy rain.

The other options are incorrect because:

• A. Slightly is not a strong enough word to describe the
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  drastic rise in the river level.
• B. Suddenly is not a
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  precise enough word to describe the rise in the river level. It is possible that the river level rose gradually, but then suddenly increased at a faster rate.
• D. Unexpectedly is not a necessary word to describe the rise in the river level. It is possible that the rise in the river level was expected, but it was still drastic.

The river level was rising greatly after that heavy rain. This means that the river was getting much higher very quickly. This could be dangerous, as it could cause flooding.