Which of the following statement (s) are true? a. a low 500 mb height indicates denser air below, and more atmosphere and lower pressure above. b. a low 500 mb height indicates denser air below, and less atmosphere and higher pressure above. c. a low 500 mb height indicates denser air below, and less atmosphere and lower pressure above. d. a high 500 mb height indicates denser air below, and less atmosphere and lower pressure above. e. all the statements above are correct