题目

 This photo brought my care-free childhood back to my mind       I would spend long summer evenings playing on the beach. A. when                        B. where                      C. that                   D. which 答案：A纳米是长度单位，纳米技术已广泛应用于各个领域．已知1纳米=0.000 000 001米，一个氢原子的直径大约是0.1纳米，用科学记数法表示一个氢原子的直径约为________米．