From the given quotation samples, select the in$-$text quotation that is correct using punctuation, capitalization, and format?

a$.$ He stated, "Students often had difficulty using APA style in college" $($Mitchell$,2003,$ p$.9),$ but he did not offer an explanation as to why.

b$.$ He stated, "students often had difficulty using APA style in college" $($Mitchell$,2003),$ but he did not offer an explanation as to why.

c$.$ He stated, "Students often had difficulty using APA style in college" $($Mitchell$,$ p$.9),$ but he did not offer an explanation as to why.

d$.$ He stated, Students often had difficulty using APA style in college $($Mitchell$),$ but he did not offer an explanation as to why.