果树枝接的时期是在（）
参考答案：春季（树液开始流动后.接穗芽未萌动时）
Along with the letter was his promise _______ he would visit me this coming Christmas. [ ]
A. which
B. that
C. what
D. whether
平面汇交力系的力多边形如图所示，该力系的合力等于：().
A.A
B.B
C.C
D.D
的力多边形如图所示，该力系的合力
等于：().
