<h2>Answer:</h2>
Compound. Option A is correct
<h2>Explanations:</h2>
When two or more different elements combines, a compound is formed. For instance, if carbon and oxygen combines, carbondioxide is formed according to the equation;

From the reaction, carbon and oxygen are the elements while carbondioxide is the compound. Hence we can conclude that a pure substance formed when two or more different elements combine called compound.
Answer:
1400000 cm
Explanation:
14 km equals to 1400000 cm (14 km = 1400000 cm)
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Answer:
30 g of magnesium would be combined with 20 g of oxygen. The law used solving this problem is the Lavoisier Law of conservation of mass.
Explanation:
If 60 g of magnesium combines with 40 g of oxygen to make 100 g of magnesium oxide, then 30 g of magnesium will combine with 20 g of oxygen to make 50 g of magnesium oxide.
This happens because in a chemical reaction there is no creation or descruction of atoms, only a rearrangement. Therefore, the mass of reactants should be equal to the mass of products.
The following equation represents the proportions of the substances:
Mg + 1/2O₂ → MgO
These are main components of Industrial Smog. Smog is air pollutant. Main sources of Smog are 1) Coal, 2) Transportation Emission, 3) Natural Causes. Those areas are mainly effected which are located in Industrial Zones and those which has high traffic density. Cities like Delhi, Beijing, London, Maxico City, Log Angeles, Tehran e.t.c are reported to be highly effected by smog. Among these polutants, CO₂ is the main source of Global Warming, CO has adverse effects on human health as it has 200 times more potency to bind with Hemoglobin compared to Oxygen, SO₂ when present in air reacts with water producing acid rain e.t.c.
CaCl2
All of the other choices are elements