个人外汇买卖业务一般本着钞变钞、汇变汇的原则，以下说法正确的是( )。

A．现钞可以随意兑换成现汇

B．现钞不能随意兑换成现汇，需要支付一定的钞变汇手续费

C．现汇买入价与现钞买入价总是相同的

D．现汇卖出价与现钞卖出价总是不同的

Certificates are （71） documents attesting to the （72） 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 （73） someone else. In their simplest form, Certificates contain a public key and a name. As commonly used, a Certificate also contains an （74） date, the name of the CA that issued the Certificate, a serial number, and perhaps other information. Most importantly, it contains the digital （75） 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．text

B．data

C．digital

D．structured