China is one of the countries with the ____ (long) history.
五氧化二碘(I2O5)可用来测定空气中一氧化碳的含量,中碘元素的化合价为( )
如图所示,用轻质杆将一电灯吊起,O点是支点.请画出动力F1的力臂L1和阻力F2的示意图.
____