1
Explanation:
The value of all conversion factors that are used in dimensional analysis must be 1.
Dimensional analysis is a beautiful and systematic way of converting one unit to another using a simple scientific method in which a conversion factor of 1 is used to multiply the number to be converted.
For example, let us convert 1200kg to g:
We know that;
1000g = 1kg
The conversion factor is;

The above value is the same as 1 since 1kg and 1000g are the same.
Now multiply with 1200kg;
1200kg x
= 1200000g
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The answer to your question is oxygen; prevent
Do you mean; " A blank is a substance that is made by combining two elements?" (I'm not sure if substances is the right term)
If so then the blank is a compound.
Answer:
eye weakness is caused by deficiency of vitamin A