图为东亚部分地区两日天气图，图中H为高气压、L为低气压。读图回答下列问题。（13分）

（1）9月21日赣州的风向是_____，说明赣州两日风力大小的变化及判断理由。（4分）

（2）除风力变化外，赣州20、21日的天气状况还发生哪些变化？并分析产生变化的原因。（6分）

（3）若L1发展强烈，会导致我国福建省洪涝灾害严重，试分析其原因。（3分）

Certificates are (16) documents attesting to the (17) of a public key to an individual or other entity. They allow verification of the claim that a given public key does in fact belong to a given individual. Certificates help prevent someone from using a phony key to (18) someone else. In their simplest form, Certificates contain a public key and a name. As commonly used, a certificate also contains an (19) date, the name of the CA that issued the certificate, a serial number, and perhaps other information. Most importantly, it contains the digital (20) of the certificate issuer. The most widely accepted format for certificates is X.509, thus, Certificates can be read or written by any application complying with X.509.

A．signature

B．mark

C．stamp

D．hypertext