—Hi, Maria, you look tired.
—I am tired. I ______ on my homework all day.

(单选题)21.（ ） (单选题)What to be Stephen Long’s attitude towards the study? (单选题)How does the plant’s protective system working according to (单选题)What will happen to a plant when exposed to too much sunligh (单选题)The closest meaning of the underlined word in Paragraph 2 is (单选题)What would be the best title for the text? (单选题)It can be learned from the text that ______. (单选题)The example mentioned in Paragraph 6 is used to show ______. (单选题)Why do experts think emotional machines won’t exist? (单选题)The last two paragraphs mainly imply that ______.