MCAT chemistry/physics question

[holdmystethoscope]

This question is from one of my practice exam. The explanation isn't so great I think. Green is the correct answer. Any help would be appreciated.

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[SOOyC52H1Sof]

So its basically saying at time A, the neuron is at resting membrane potential. At time B, there had to be a positive current to get it past resting membrane potential.

So between time A and B, something had to happen in order to make the voltage inside the cell more negative. This could only happen if positive ions move our or negative ions move in between time A and B.