Which xDSL flavour delivers both downstream and upstream speeds of 1.544 Mbps over two
copper twisted pairs?

A.
HDSL

B.
SDSL

C.
ADSL

D.
VDSL

Explanation:
HDSL – High-Data-Rate Digital Subscriber Line – 1.544 Mbps each way over 2 copper twisted pair
(http://www.cisco.com/en/US/tech/tk175/tk318/tsd_technology_support_protocol_home.html)