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1.                             Interest rates on mortgages have declined steadily during the first six months of this year but virtually remained unchanged during the next three months.fficeffice" />

(A) have declined steadily during the first six months of this year but virtually remained unchanged

(B) declined steadily during the first six months of this year but virtually remain unchanged

(C) steadily declined during the first six months of this year but remain virtually unchanged

(D) declined steadily during the first six months of this but have remained virtually unchanging’（E）

(E) declined steadily during the first six months of this year but have remained virtually unchanged

why past tense and present perfect here? thanks