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Read the local sheriff's report about a strange light in the sky and choose the most logical deductions.
Old Man Jenkins claims he saw an alien spaceship, but he didn't have his glasses on, so he _________________________ easily mistaken a weather balloon for a UFO. Furthermore, the "crop circle" in his field has deep tractor tire tracks right through the middle, so extraterrestrials _________________________ created it!
The correct answers are could have and can't have.
Could have expresses a past possibility. Because he wasn't wearing glasses, it's very possible he made a mistake.
Can't have expresses a strong negative deduction. The tractor tracks provide undeniable proof that aliens were not responsible for the crop circle.