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Check the picture below.
Answer:
The correct option is C: the isotope of iodine contains 81 neutrons.
Step-by-step explanation:
The mass (A) of the iodine isotope is given by:

Where:
Z: is the number of protons
N: is the number of neutrons
For iodine Z= 53, so if A = 134 the number of neutrons is:

Therefore, the correct option is C. The isotope of iodine contains 81 neutrons.
I hope it helps you!
There is 15 bars in each case.
Subtract 8 from 143 and you get 135.
Divide 135 by 9 and you get 15 :)