__________was one of the most difficult tasks pioneers faced on their journeys west.Đoạn văn61/65IV. WRITING a. From the words or phrases - a, b, c or d - choose the one that best completes the sentences.__________was one of the most difficult tasks pioneers faced on their journeys west.Having crossed riversWhile crossing riversCrossing riversBy crossing riversGiải thíchĐáp án đúng là c