Blank (i): The "Vastness" Clue
The sentence starts by talking about how big the universe is. We need a word that means "extent" or "stretching area."
- A. length: This is too small. "Length" usually describes a 1D line (like the length of a table).
- B. darkness: This is a quality of space, but it doesn't describe the "vastness" or "size."
- C. reaches: This is a classic GRE word for "far-reaching areas." When we say "the outer reaches of space," we are talking about its massive, 3D extent.
- Winner: reaches

Blank (ii): The "Hard to Believe" Trigger
The sentence says that even though we think the Milky Way is unique, it's actually just one of many. This makes it "hard to $\_\_\_\_$ " the scale of it.
- D. fathom: To "fathom" something means to wrap your mind around it or deeply understand its magnitude. This fits perfectly with the idea of something being "hard to believe" or "incredible."
- E. explain: We can explain it scientifically, but the sentence is about the emotional/intellectual difficulty of accepting how big it is.
- F. measure: We actually can measure parts of the universe. The sentence is about our perception, not our math.
- Winner: fathom

Blank (iii): The "Movement" Clue
The last part of the sentence specifically says the galaxies are "moving through the universe." We need a scientific way to describe that movement.
- G. velocities and trajectories: In physics, velocity is speed in a specific direction, and trajectory is the path an object follows. This is the precise scientific way to describe how galaxies move.
- H. ways and means: This is a business/legal term. It means "the methods used to achieve a goal." It has nothing to do with outer space.
- I. angles and dimensions: These describe the shape or position of an object, but they don't describe the act of moving.
- Winner: velocities and trajectories

The "Logic Lab" Summary
The passage moves from the Scale (reaches) → to the Human Reaction (fathom) → to the Scientific Fact (velocities and trajectories).