Arrange the following substances in order of increasing mass

Arrange the following substances in order of increasing mass percent of nitrogen.
(a) NO,a gas formed by the reaction of N2 with O2 in internal combustion engines.
(b) NO2,a brown gas mainly responsible for the brownish color of photochemical smog.
(c) N2O4,a colorless liquid used as a fuel in space shuttles.
(d) N2O,a colorless gas sometimes used as an anesthetic by dentists (known as laughing gas).
NO2 = N2O4 < NO = N2O
NO2 = N2O4 < NO < N2O
NO2 < N2O4 = NO < N2O
NO2 < N2O4 < NO < N2O
（以上不等式哪个正确?）