The author implies that bacteria were investigated earlier than viruses

* Hướng dẫn giải

Chọn đáp án A

Thông tin trong bài: 'The nature of viruses made them impossible to detect for many years even after bacteria had been discovered and studied.Not only are viruses too small to be seen with a light microscope, they also cannot be detectedthrough their biological activity, except as it occurs in conjunction with other organisms."