以观赏特性分类:
参考答案:
花篱类,如金丝桃、紫荆、瑞香、木槿、金钟花、杜鹃等;果篱类,如南天竹、火棘、栒子等;刺篱类,如小檗、柞木、枳壳、云实、枸骨等。
_____ many times, John still couldn't work out the problem. [ ]
A. Trying
B. Tried
C. To try
D. Having tried
引起雷诺现象的主要振动频率为()
A.40Hz以上
B.40Hz以下
C.40~300Hz
D.40~800Hz
E.16~1500Hz