Which  of the levels: A1 A2 B1 B2 C1 best describes the difficulty of text: In terms of this model the critical points are the junctions at which the lines fork, for once a train is on any particular line, pressures are present that keep it on that line; although, provided divergence does not become too great, there remains an opportunity for a train to take a convergent track when the next junction is reached.
b1