Writing help, verb tense confusion

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1

<p>Radioactivity is generally not considered harmful when people are exposed to it at low levels for brief periods, but less clear is its long-term effects.</p>

<p>" IS" is the error but isnt the subject of the sentence radioactivity? if so isnt it singular?</p>

2

<p>Which “is”? (yes, I know which “is”, but do you?)</p>

3

<p>but less clear is its long-term effects.
Rephrased => Its effects is(are) less clear</p>