Why do we choose "sometimes"?

[belly_ttt]

Pistachios grow on trees in grape-like clusters and are encased in an outer skin, or hull. When pistachios ripen, the hull turns rosy and the shell within splits naturally, indicating they are ready for harvest, usually in September.
California pistachios are mechanically shaken from the trees onto a catching frame, never touching the ground. Each tree takes less than a minute. They are loaded onto containers and rushed to the processing plant where hulling, drying and sorting are completed. Later the nuts are roasted, salted and _______ dyed.
a) sometimes
b) never
c) thus
d) very


I wonder why we choose a here although b could make sense (from the ecpe exam)​

 

[shalstudent]

I feel "sometimes" makes sense. "Never" would sound better when "but" is used.

--Later the nuts are roasted, salted but never dyed.

--Later, the nuts are roasted, salted and sometimes dyed. (only dyed sometimes, not always).

 

[Wuisi]

I do agree with shalstudent in that 'never' there would imply some kind of contrast and so require 'but' instead of 'and'.