Solution

Sentence B says that scientists have identified an unknown salivary gland. Sentence D connects it and says that the discovery is the first major gland found in 300 years and have paved the way for techniques to increase quality of life of people undergoing cancer treatment. So, B-D becomes a pair. Sentence C says about sparing the glands in radiotherapy which could reduce difficulty. The sentence is completed by E which states the difficulty which may happen during salivation and swallowing in head and neck cancer patients. So, C-E becomes a pair. Therefore, option B is the right answer.