下面表示追加写功能的语句是（ ）。 A: fp_file=fopen(str,"wb");ufd[k]->ufdfile[i].flength=WriteF1(); B: fp_file=fopen(str,"ab");ufd[k]->ufdfile[i].flength=ufd[k]->ufdfile[i].flength+WriteF1(); C: fp_file=fopen(str,"wb");ufd[k]->ufdfile[i].flength=ufd[k]->ufdfile[i].flength+WriteF1(); D: fp_file=fopen(str,"wb");ufd[k]->ufdfile[i].flength=WriteF1();