A、It chooses values for one variable at a time and backtracks when a variable has no legal values left to assign. 每次为变量选择值并且当变量没有合法赋值时回溯。
B、It incrementally builds candidates to the solutions, and abandons each partial candidate c, as soon as it determines that c cannot possibly be completed to a valid solution. 每次为变量选择值并且当变量有一个或多个合法赋值时回溯。
C、It incrementally builds candidates to the solutions, and abandons each partial candidate c, as soon as it determines that c cannot possibly be completed to an invalid solution. 递增地构建解的候选，并且一旦确定部分候选c不能成为合法的解，就将c抛弃。
D、It chooses values for one variable at a time and backtracks when a variable has one more legal value left to assign. 递增地构建解的候选，并且一旦确定部分候选c不能成为非法的解，就将c抛弃。