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If the total of the resistor voltage drops in the shown loop is $18\,\text{V}$, what must the source voltage $V_s$ be according to Kirchhoff's Voltage Law? 
A$-18\,\text{V}$
B$0\,\text{V}$
C$18\,\text{V}$
DIt cannot be determined
Answer & Solution
Correct answer: C. $18\,\text{V}$
From the relation $V_1+V_2+V_3+V_4+V_5+V_6+V_7-V_s=0$, we get $V_s$ equal to the sum of the drops. So if the total drop is $18\,\text{V}$, then $V_s=18\,\text{V}$.