目前我国境内已知的最早的人类是（　　）

    I live in a nice new building in Beijing. It is very tall. It has eighteen floors. There are about eighty families in the building. That is to say (也就是说), about two hundred and forty people live in the building.

    My home is on the twelfth floor. We have two bedrooms and a living room. They are big and clean. There are a lot of big trees and beautiful flowers around our building. I like looking out of the window in my room. I can see cars and buses running in the street. I can see people walking in the street, too. They look so small! I can also see a lot of tall buildings. How beautiful Beijing is! I am happy to live in this nice, new and tall building.

长木板A放在光滑的水平面上，质量为m的小物块B以水平初速度v₀从A的一端滑上A的水平上表面，它们在运动过程中的v－t图线如图所示。则根据图中所给出的已知数据v₀、t₁及物块质量m，可以求出的物理量是（   ）