简述单丝壳属（Sphaerotheca）特点。
参考答案：
闭囊壳内子囊，附属丝菌丝状。引起瓜类、豆类等多种植物的白粉病。
已知三角形三边满足a>b>c，且b=7，c=5，求a的整数值。
---When did you return to China?
---_________________.
A．Until next week
B．Not until next week
C．a few days ago
D．for a few weeks
