Which preposition to use with thorniest

as Occurrences 1%

An indescribable footpath wound through an inextricable labyrinth, sometimes as thorny as a heath, sometimes as miry as a marsh.

at Occurrences 1%

A woman's road to the earning of her own living, especially when she is weighted with the care of a young child, is always fairly thorny at the outset, and does not need to be rendered yet more difficult by secret attempts to injure, on the part of those who trust that suffering and poverty may avail to bend pride to submission.

for Occurrences 1%

Oh, I agree with you, Marquis, a woman who has only temper and caprices is very thorny for an acquaintance and in the end only repels.

Which preposition to use with  thorniest