Which statement about DNS is true? 

Answer options:

A. The client-server architecture is based on push-pull messages.
B. Query and response messages have different format.
C. In the DNS message header, the QR flag set to 1 indicates a query.
D. In the DNS header, an Opcode value of 2 represents a server status request.
E. In the DNS header, the Rcode value is set to 0 for format error.

D