The most effective way to vary the length of the sentences in the passage is to combine sentences 2, 3, and 4 to create a series. This approach would create a longer, more complex sentence that enhances the rhythm of the passage while maintaining clarity and flow.
For example, combining those sentences could produce something like: "Exercise strengthens muscles, helps prevent unhealthful weight gain, and may reduce the risk of some cancers." This technique effectively varies sentence length without losing the informative quality of the passage.