Câu 13: You're falling short on class attendance and you failed to submit the last assignment. You're________ in this semester.

   A. walking on thin air                                                 B. walking on thin eggshells

   C. walking on thin ice                                                D. walking on thin sky

Idiom: Walking on the thin ice: Gặp khó khăn dễ dẫn đến thất bại