Today, we are closer to fulfilling America’s promise of economic and social justice because we stand on the shoulders of giants like Dr. King ..."

What might President Obama mean by saying “we stand on the shoulders of giants like Dr. King”?
Americans are often blocked by Dr. King’s work.
Americans are building on Dr. King’s work.
Americans are unable to improve on Dr. King’s work.
Americans are ignoring Dr. King’s work.