如图为二次函数y=ax²+bx+c（a≠0）的图象，则下列说法：①a＞0  ②2a+b=0 ③a+b+c＞0 ④当﹣1＜x＜3时，y＞0其中正确的个数为(       )

一物体的质量是2.7×10⁴kg，体积为10m³，则其密度为　____ 　kg/m³，合　____ 　g/cm³，若把该物体切去一半，则剩下部分的密度将____ （选填“变大”、“变小”或“不变”）．

— In the present, most of us have no enough money to afford the houses.

  — Yes, they are too expensive in Shenzhen.