voyage (6) enquête (6) transport en commun (6) expérience scientifique (6) science économique (6) Travel (6) Travel (6) Economics (6) puits (5) recherche (5) prix (5) politique (5) combustible (5) gouvernement fédéral (5) énergie (5) demande (offre et demande) (5) Instruments (5) Weights and measures (5) Transportation (5) Spring (5) Research (5) Weight loss (5) Power resources (5) Power (Mechanics) (5) Months (5) Migration, Internal (5) Households (5) Fuel (5) Force and energy (5) Federal government (5)