OFFICIAL EXPLANATION


A

Though it is not explicitly stated in the passage, you can, from a few clues, determine that a planetesimal was (1) an object that moveda sort of wandering body-since they are described as "itinerant," and (2) an object that existed during "planet formation." Choice (A) follows from those two inferences and is correct. There is no support for choice (B); the passage makes no mention of objects leaving or bouncing out of the galaxy. You can eliminate choice (C) because the passage tells you that components of the planetesimals (the siderophiles) remained in Earth's crust after impact but doesn't say that whole planetesimals did so. Choice (D) is beyond the scope of the passage, which doesn't contain information about planetesimals' magnetic characteristics. You can also cross off ( $\mathbf{E}$ ) because the passage doesn't describe planetesimals as "rocks," nor does it claim that they are always made up of (or necessarily even contain) siderophiles.