近年来，我国为维护公平，构建社会主义和谐社会做了许多努力。下列能反映这一内容的是 [     ]

①实施农村最低生活保障制度

②解决农民工子女入学难问题

③免除“农业税”

④禁止未成年人上网

A．①②③

B．②③④

C．①③④

D．①②④

When the system upon which a transport entity is running fails and subsequently restarts,the(21) information of all active connections is lost. The affected connections become half-open, as the side that did not fail does not yet realize the problem.The still active side of a half-open connections can close the connection using a(22)timer. This timer measures the time the transport machine will continue to await an (23) of a transmitted segment after the segment has been retransmitted the maximum number of times.When the timer (24),the transport entity assumes that either the other transport entity or the intervening network has failed. As a result, the timer closes the connection, and signals an abnormal close to the TS user. In the event that a transport entity fails and quickly restarts, half-open connections can be terminated more quickly by the use of the RST segment, the failed side returns an RST i to every segment i that it receives. When the RST i reaches the other side, it must be checked for validity based on the (25) number i, as the RST could be in response to an old segment. If the reset is valid, the transport entity performs an abnormal termination.

空白处（21）应选择（）

A.data

B.state

C.signal

D.control