For sentence-completion inference questions, first restate what the blank must explain in your own words (here: why balloons can’t achieve real-time, continent-scale coverage). Then choose the option that most directly restates a limitation explicitly supported by the passage, avoiding answers that add new claims or purposes not mentioned.
Hints
- Hint 1
Restate the contrast
The new method gives “real-time measurements across entire continents.” What specific limitation of balloons would prevent that scale of geographic coverage?
- Hint 2
Use the sentence about “sparingly”
The passage directly says balloon flights were conducted sparingly. What does “sparingly” imply about how many places and how often data could be collected?
- Hint 3
Prefer choices that stay within the passage
Eliminate choices that add new purposes (like calibration) or claims about real-time transmission that the passage doesn’t state.
Step-by-step
- Step 1
Identify what the blank must do
The blank must explain why the balloon method could not provide “real-time measurements across entire continents.” So the completion should point to a limitation that prevents wide, continuous spatial coverage.
- Step 2
Pull the key balloon details from the text
The passage says each balloon launch cost several thousand dollars and produced only one vertical profile, so flights were conducted sparingly. That implies limited frequency and limited geographic coverage.
- Step 3
Match the limitation to continent-scale coverage
To measure across entire continents in real time, researchers would need many measurements from many places, collected continuously or very frequently. Sparse balloon launches from relatively few locations cannot meet that requirement.
- Step 4
Confirm the best completion
Therefore, the best completion is: they cannot be launched often enough from enough locations to achieve such spatial coverage.