Many parents are understandably concerned that it is dangerous to start serious training in a sport (4) ______ an early age.Đoạn văn24/30Many parents are understandably concerned that it is dangerous to start serious training in a sport (4) ______ an early age. at in from onGiải thíchĐáp án AGiải thích: at an early age: ở độ tuổi nhỏ