题目:设函数f(x)在[a,b]上连续,在(a,b)内可导,且f′(x)<0,f(a)=f(b)=0,则根据介值定理,存在c∈(a,b)使得( )。A. f(c)=0B. f′(c)=0C. f(c)>0D. f(c)<0