Mark the letter A, B, C, or D on your answer sheet to indicate the cor

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the following questions.    

Experts agree that this is due to urban development, when open green spaces are replaced with asphalt roads and tall brick or concrete buildings. These materials retain heat generated by the Sun and release it through the night. In Atlanta, in the US, this has even led to thunderstorms  ______ occur in the morning rather than, as is more common, in the afternoon.  

• A. A. which
• B. B. where
• C. C. what
• D. D. when

Đại từ quan hệ “which” thay thế cho danh từ “thunderstorms” đứng trước nó để có mệnh đề quan hệ hoàn chỉnh “thunderstorms which occur in the morning rather than…”.