<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.
The field of study that Christian Doppler's work influenced the most was about <u>light </u><u>waves.</u>
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Answer:
6.46 × 10⁻¹¹ M
Explanation:
Step 1: Given data
pH of the solution: 3.81
Step 2: Calculate the pOH of the solution
We will use the following expression.
pH + pOH = 14.00
pOH = 14.00 - pH = 14.00 - 3.81 = 10.19
Step 3: Calculate the concentration of OH⁻ ions
We will use the definition of pOH.
pOH = -log [OH⁻]
[OH⁻] = antilog -pOH = antilog -10.19 = 6.46 × 10⁻¹¹ M