Do you know what is meant by needed? does it mean lone pairs?
1) Carbon-13:
Proton-6 Neutron-7 Electron-6
2)Atomic mass of element X:
(55*10+56*20+57*70)/100=56.6
Total density of filled ball with nitrogen gas: 
The relationship between mass and volume can be easily determined using density; for example, the mass of a body is equal to its volume multiplied by the density (M = Vd), whereas the volume is equal to the mass divided by the density (V = M/d). The ball filled with nitrogen will not float in the air because total density of filled ball is greater than the density of an air. Density of the evacuated ball D = 0.214 g/L
Density of nitrogen gas = 
Mass of the nitrogen gas : 
Learn more about Mass and Density here:
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It should be D, cells are parts, tissue is part and organ is part of a system.
Answer:
Thermometer A, because it measures accurately to the tenths digit.