Questions

If an object moves with some velocity that means the magnitude and direction of the object changes i.e, the object moves with some acceleration. Hence, if the object moves with constant velocity, no change in direction and magnitude of speed i.e, no acceleration = 0 acceleration.

When water is cooled from 4°C to 0°C, its density decreases.

When water cools from 4°C to 0°C, ice crystals begin to form because the water is approaching its freezing point. When water freezes, the water molecules form a lattice that increases the volume. As a result, the density decreases. This is why ice will float on top of water; ice has a lower density than the water.

PTH - Parathyroid hormone which balances calcium levels if they are low.

System calls provide the interface between a process and the operating system. System calls for modern Microsoft windows platforms are part of the win32 API, which is available for all the compilers written for Microsoft windows.

A chemical reaction in which involves the absorption of heat from the environment is called a Endothermic reaction. 

Examples : Dissolving ammonium chloride in water, photosynthesis, melting ice cubes, mixing of potassium chloride with water,...

In the contrast, chemical reactions involving release of heat are known as exothermic reactions.

Examples : Combustion, coal is burnt, mix of CaO with water,...