Peter was so excited _______ he received the invitation to the important meeting.
A:that
B:though
C:when
D:before
How kind you are! You always do what you can __________ others.
如图所示,在天花板上悬挂一轻质弹簧,当在弹簧下端挂上质量m1=0.5kg的重物时,弹簧的长度L1=22cm;当在弹簧下端挂上质量m2=0.75kg的重物时,弹簧的长度L2=23cm.已知弹簧始终在弹性限度内,取g=10m/s2,求该弹簧的劲度系数k.
关于非洲的叙述,不正确的是( )