The most reliable way to detect the presence of life on a planet would be by determining whether or not its atmosphere contains methane. This is because methane completely disappears from a planet' s atmosphere through various chemical reactions unless it is constantly replenished by the biological processes of living beings. Which one of the following statements, if true, most seriously weakens the argument? ['Not all living beings have the ability to biologically produce methane.', 'Earth is the only planet whose atmosphere is known to contain methane.', 'There are other ways of detecting the presence of life on a planet.', 'Some living beings biologically produce only very small amounts of methane.']