What's the Theme?

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Across
  1. 2. coffee, tea, milk, Bubble-tea
  2. 4. shirts, pants, skirts, blouse
  3. 7. addition, subtraction, multiplication, division
  4. 9. Iron Man, Captain America, Thor, Black Widow
  5. 10. tennis, swimming, cycling, rock-climbing
  6. 11. cats, dogs, hamster, terrapin
  7. 12. January, March, May, November
Down
  1. 1. Jupiter, Saturn, Venus, Neptune
  2. 3. sneakers, pumps, sandals, flip-flops
  3. 5. summer, spring, autumn, winter
  4. 6. Ford, Mercedes, Hyundai, Toyota
  5. 8. daisies, lilies, tulips, carnations