问题补充：
on the grass singing and laughing.
A. Seated ; is some students
B. Seated ; are some students
C. Sitting ; is some students
D. Sitting ; some students are
答案：
A. Seated ; is some students
(答案→)B. Seated ; are some students
C. Sitting ; is some students
D. Sitting ; some students are
解析：句意：坐在草地上，这些学生又是唱又是笑。这是倒装句，主语是some students所以谓语动词用复数，正常的语序是sb be seated on the grass singing and laughing.故选B。
