Scientists photographed for the first time ever _____ wild gorillas used a stick to test the depth of a pool before getting into it, according to a recent study.

1. A.
   which
2. B.
   what
3. C.
   that
4. D.
   when

Scientists photographed for the first time ever _____ wild gorillas used a stick to test the depth of a pool before getting into it, according to a recent study.

A. which             B. what               C. that               D. when

Scientists photographed for the first time ever _____ wild gorillas used a stick to test the depth of a pool before getting into it, according to a recent study.

A. which             B. what               C. that               D. when

Scientists photographed for the first time ever _____ wild gorillas used a stick to test the depth of a pool before getting into it, according to a recent study.

A. which           B. what             C. that             D. when

Scientists photographed for the first time ever _____ wild gorillas used a stick to test the depth of a pool before getting into it, according to a recent study.

A. which           B. what             C. that             D. when

Scientists photographed for the first time ever _____ wild gorillas used a stick to test the depth of a pool before getting into it, according to a recent study.

A. which　　 　 B. what　　 　 C. that　　　 　 D. when

Scientists photographed for the first time ever _____ wild gorillas used a stick to test the depth of a pool before getting into it, according to a recent study.

A. which　　 B. what　　 C. that　　　 D. when

Scientists photographed for the first time ever      wild gorillas used a stick to test the depth of a pool before getting into it, according to a recent study.

A．which           B．what           C．that           D．when