It is along the Mississippi River ---- Mark Twain spent much

It is along the Mississippi River ____ Mark Twain spent much of his childhood.A.how B.which C.that D.where____ a certain doubt among the people as to the practial value of project.A.It has B.They have C.It remains D.They remainsIt _____ John and Mike who cleaned the room.A.are B.were C.was D.has been Will you mend the cleaner yourself ,______ shall we ask someone else to fix it A.and B.so C.or D.for答案CDCC需要解释原因