心脏病孕妇最容易发生心力衰竭的时期是（）

A．妊娠20～22周

B．妊娠24～26周

C．妊娠28～30周

D．妊娠32～34周

E．妊娠36～38周

A typical application of this (71) is ADSL. It is emerging as the technology for home-and small-office Internet connectivity. It provides either 1.5 Mb/s～8 Mb/s from the network to the user and 64 Kb/s～512 Kb/s in the reverse direction depending on the distance, 12 000 or 18 000 feet.. The different speed for each direction gives it the (72) label. ADSL is designed to take advantage of the fact that video-on demand, telecommuting, and Internet access traffic are inherently asymmetrical. The user (73) a brief message up to the network and receives a ton of data coming back, either a movie or a piece of data download. Under such a scenario, low-speed traffic to the network is just fine. ADSL delivers high (74) where you need it and only uses a single copper pair. Through multiplexing, it also reserves (75) of the bandwidth for POTS.

A．2kHz

B．4kHz

C．16kHz

D．64kHz