People do not know for sure when numbers came into use. It is not______for sure when numbers came into use.
举一反三
- an expression people use when trying something they are not sure will be successful._______
- —Do you know ? —Yes. He came here by train. A: when he came here B: if he will come here C: why he came here D: how he came here
- When writing the main body, you must _______. A: Look closely at the diagram and make sure you understand the information being show B: Look for units C: Think about which prepositions to use. D: Think about what verb tense to use.
- People are sure to die when infected with COVID-19. ( )
- What do we know about credit cards according to the passage（） A: They are safe to use. B: They are convenient to use. C: They are economical to use. D: They are complicated to use.