The first compact microwave ovens were sold in 1960, but everybody didn’t use them until the 1980’s.
But is used to connect the 2 sentence because the two sentences give contradicting ideas
Microwave oven was sold during 1960 however people did not use it until the 1980s.
According to the rule of world language, if two given sentences gives idea with contradiction, you use but, yet, however, etc.
For example:
=> I bought a power bank online but I did not received it.
=> I love sweets however too much is yucky.
I think
D ;
The best conjunction to complete the sentence is "but," which correctly contrasts the availability of microwave ovens in 1960 with their later usage in the 1980s. This selection highlights the opposing ideas of availability versus popularity. Thus, the full sentence reads: "The first compact microwave ovens were sold in 1960, but everybody didn’t use them until the 1980’s."
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