题目

Philadelphia, Pennsylvania is America’s fifth largest city. Once a major American colonial city, it is the home of America’s first library, its first hospital, and its first zoo. Now it is also the first US city to be named a World Heritage City. On November 6, 2015, Philadelphia joined more than 260 other cities that have been recognized for their influence on the world. These cities include Paris, France, Florence, Italy, and Cairo, Egypt. “Today marks the start of a new and exciting chapter in the history of Philadelphia,” remarked Philadelphia Mayor Michael Nutter when the anno如图所示，由不同物质制成的甲、乙两种实心球的体积相等，此时天平平衡。则制成甲、乙两种球的物质密度之比为（ ）A. 3︰4 B. 4︰3 C. 2︰1 D. 1︰2