4.15There’s plenty of time, so you ()worry about it.

A. 　 as to be heard

B. 　 to be heard

C. 　 as to hear

D. 　 to hear

A. 　 go off

B. 　 go by

C. 　 go over

D. 　 go against

A. 　维管束的营养供应,要靠植物的木射线来完成,这一运输通道的连接也成了树木生长的一部分。

B. 　因为存在着被称为“桥梁”的维管束和“桥间之桥”的木射线,树木才能够不断地生长壮大。

C. 　维管束的使用寿命一般都很短,不断被废弃。废弃的维管束如果过多,会影响树木的正常生长。

D. 　维管束有一定的使用年限,而形成层能够不断制造出新的维管束,从而支撑着树木生命的延续。

A. 　looked

B. 　 sat

C. 　 travelled

D. 　 lived

A. 　 had begun

B. 　 will begin

C. 　 has begun

D. 　 begins

A. 　really

B. 　 almost

C. 　 hardly

D. 　 still

A. 　 check

B. 　 explain

C. 　 describe

D. 　 mark

A. 　 bright

B. 　 weak

C. 　 soft

D. 　 natural

A. 　address

B. 　 word

C. 　 area

D. 　 picture