Should it be "persistent [high] levels of unemployment" or "persistently high levels of unemployment"?

Here is a sentence that I wrote for an academic paper:

… the provision of longer benefit durations in regions of high unemployment creates long-term EI dependency among seasonal workers, which then results in persistent high levels of unemployment in those regions.

Question: Which phrasing is the most idiomatic (correct usage)? My research tells me I should go with C (but my research is based on the graph, google hits, and reading some of the lines on those hits). However, google hits for all three options are similar in number: around 130.

• A. persistent levels of unemployment
• B. persistent high levels of
  unemployment
• C. persistently high levels of unemployment

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Details:

Let’s start with the definition: according to Cambridge, "persistent" (adj) means

lasting for a long time or difficult to get rid of

The intended meaning is that unemployment in those regions stay up high year after year, as seasonal workers don’t relocate to other regions for new jobs. They prefer being unemployed in the high-unemployment regions so they can get unemployment benefits for longer durations (benefit durations are shorter in low unemployment regions).

My original phrasing was option A, "persistent levels of unemployment", which turned out to be very unpopular/uncommon. "Level" is like a scale ranging from low to high. I wonder if that is the problem: "persistent" and "level" does not work together.

In google news, there are only

• A. 4 entries for persistent levels of unemployment

• B. 11 entries for persistent high levels of unemployment

• C. 24 entries for persistently high levels of unemployment

Simply looking up google all reveals around 130 entries for all of the above options (google returns around 10,300 entries, but if you click page 15 or so, you will see google revises its estimate to around 130 entries). Provided is an ngram for comparison. One term that is way up higher than all three is just "persistent unemployment" (not in image). I wonder why the green (A) and the red (B) line cross like that.