21. Many people feel more relaxed riding their bikes along the country roads than ______ in big cities.
(A) bikes
(B) biking
(C) to bike
(D) go biking

Many people feel more relaxed riding their bikes along the country roads than ______ in big cities.  (A) bikes 

(B) biking

(C) to bike

(D) go biking

more than 自身兩種特性的比較，前後詞性要相同；riding their bikes = biking 都是「騎自行車」的意思 故選(B)

個人見解，有誤請指教更正

more和than的分離：

more than如果分開，than前後是兩個並列平行的成分，那麼該結構表示一種「取捨關係」，than前面的成分就是「取」，後面的成分就是「舍」。可譯為「是A不是B」，或者「與其說是B，不如說是A」.

Many people feel more relaxed riding their bikes along the country roads than __biking____ in big cities.

許多人覺得在鄉間小路上騎自行車比在大城市騎自行車更輕鬆。