Before 1986 physicists believed they could describe the universe in terms of four universal forces. Experiments then suggested, however, a fifth universal force of mutual repulsion between particles of matter. This fifth force would explain the occurrence in the experiments of a smaller measurement of the gravitational attraction between bodies than the established theory predicted. Which one of the following, if true, most strengthens the argument that there is a fifth universal force? ['No previously established scientific results are incompatible with the notion of a fifth universal force.', 'The extremely sophisticated equipment used for the experiments was not available to physicists before the 1970s.', 'The experiments were conducted by physicists in remote geological settings in which factors affecting the force of gravity could not be measured with any degree of precision.', 'Some scientists have suggested that the alleged fifth universal force is an aspect of gravity rather than being fundamental in itself.']