When grading student papers, Professor Arthur uses a grading rubric that specifies performance standards for grammar, organization, references, and creativity. However, rather than grading student papers based on the rubric, he tends to grade a paper by comparing it to the previously read paper. Professor Arthur's grading is most likely subject to $\_\_\_\_\_.$

a$.$ sampling error

b$.$ the contrast error

c$.$ the halo effect

d$.$ the horns effect