Many of today’s mathematicians use computers to test cases that are either too time-consuming or involve too many variables to test manually, allowing the exploration of theoretical issues that were impossible to test a generation ago
(a) are either too time-consuming or involve too many variables to test manually, allowing the exploration of
(b) either take too much time or involve too many variables to be tested manually; allowing the mathematicians to explore
(c) would either take too much time or involve too many variables to test manually, allowing them to explore
(d) would either be too time-consuming or would involve too many variables to test manually; this capability allows the mathematicians to explore
(e) take too much time or variables to test manually; this capability allows the mathematicians to explore