Which of the following statements is not in line with academic writing conventions:
选项：

A:A recent study investigated whether that the combination can help prevent mild symptoms from developing into severe and critical conditions, thus reducing the death rate of the disease.;
B:A recent study demonstrated whether that the combination can help prevent mild symptoms from developing into severe and critical conditions, thus reducing the death rate of the disease.;
C:A recent study showed whether that the combination can help prevent mild symptoms from developing into severe and critical conditions, thus reducing the death rate of the disease.;
D:all these sentences are wrong answers