Use the map below to answer the following question: Map of Europe, North Africa, and Western Asia labeled, Growth of Rome 500 BCE - 107 CE. The darkest green color is labeled, Original Roman Republic (500 BCE). The darkest green area on the map is the small area around the city of Rome. Another green color is labeled, Imperial gains by 264 BC. This green color on the map is most of the Italian peninsula. Another green color is labeled, Imperial gains by 146 BCE. This color on the map includes most of Spain, Sicily and Sardinia, Greece, and part of Northern Africa around Carthage. A light green color is labeled, Imperial gains by 44 BC. The light green area on the map extends to Gaul, Asia Minor, Syria, and Numidia in North Africa. A light yellow color on the map is labeled, Extent of Roman Empire by 107 AD. This color on the map includes all of the previous areas, an extends to Britain, as well as further inland in Europe and Asia, and the entire North African coast.
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Which statement explains why Britain became a part of the Roman Empire much later than nearby Gaul?