Current I enters junction P and splits into two branch currents I1 (downward) and I2 (to the right). These two currents later arrive at junction Q. An additional current I3 enters junction Q from above, and the combined current leaves Q to the right as I4. No charge accumulates anywhere in the circuit. Which set of equations is required for the currents to satisfy conservation of charge at both junctions?
a) At P: I=I1+I2
At Q: I1+I2=I4
b) At P: I=I1+I2
At Q: I1+I2+I3=I4
c) At P: I1=I+I2
At Q: I1+I2=I3+I4
d) At P: I=I1−I2
At Q: I1−I2+I3=I4