lt's fast to travel by underground.lt's safe to travel by underground as well.(合并成一句)
问题描述：
lt's fast to travel by underground.lt's safe to travel by underground as well.(合并成一句)
lt's ______ _______ fast ______ ______ safe to travel by underground.
答
not onlybut also为什么不但 而且的意思呀