21. Waves of red light are about twice as long as of blue light.
A. that B. these C. those D. it
22. Mark Twain’s The Adventures of Huckleberry Finn, one of the great American novels, ________ first published in the United States in 1885.
A. was B. were C. has been D. had been
23. It was only when Tom ate Zongzi for a second time to appreciate its deliciousness.
A. did he begin B. that he began C. then he began D. had he begun
24. —What do you think made Mary so excited?
— the annular solar eclipse(日环食).
A. As she watched B. Watched
C. Watching D. Because of watching
