You should be prepared for a train to appear at a railroad crossing a. between 2 a. m. and 6 a. m. b. between 6 p. m. and 10 p. m. c. at any time d. at night

Certain gases in air are important natural resources that are used in welding. two of these gases are a. oxygen and argon b. hydrogen and fluorine c. chlorine and sulfur dioxide d. nitrogen and carbon dioxide