Posting the OE - This sentence uses four phrases to describe the warning signs for an earthquake. These phrases should be parallel. The first sign is sudden fluctuations in local seismic activity; the second is tilting and other deformations of the Earth's crust. Tilting in this case is used as a noun, just as deformations and fluctuations are nouns. The first two signs are parallel. The third and fourth warning signs resemble tilting in the ing form, but they are not parallel because they are
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