Syntax highlighting for R code chunks in HTML file

Problem
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In the knitted HTML output the R code chunks are not highlighted.

Issue
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Calling knit2html() with the options as an empty string changes
the default options. That is

    knit2html("file.Rmd", options="")   # No syntax highlight
    knit2html("file.Rmd")               # Yes to Syntax Highlight

Solution
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Remove the options argument.

Effect
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Highlights R code in knitted HTML file. As desired.