In the sentences, certain words are in bold and numbered from A to H, which are the possible pairs to be interchanged. Choose the pair(s) of words that need(s) to be interchanged to make the sentence grammatically correct and meaningful.

a. The Silk Road not only serious (A) as a major trade route between (B) India and China but is also credited (C) for facilitating the spread of Buddhism from (D) India to East Asia.
b. Insomnia is becoming a very served (E) disease these days because of the increasing (F) levels of stress among (G) the young generation due to lack of employment opportunities (H)