we don't( )time to shapen the axe
A COST
B spare
为什么选B不是A呢

cost 不能用在这里
具体的看下列各自的用法

cost
AHD:[kôst]
D.J.[k%8st]
K.K.[k%st]
n.（名词）
An amount paid or required in payment for a purchase; a price.
费用：为购买而付出的或要求的数量；价格
The expenditure of something, such as time or labor, necessary for the attainment of a goal.See Synonyms at price
成本：某物的花费，例如时间或劳动，是为达到某一目的而必需的参见 price
costs Law The charges fixed for litigation, often payable by the losing party.
costs 【法律】 诉讼费：通常由失败付出的固定的讼费
n.attributive.（定语名词）
Often used to modify another noun:
通常用来修饰另一个名词：
cost consciousness; cost overruns.
成本意识；成本增量
v.（动词）
cost,cost.ing,costs
v.intr.（不及物动词）
To require a specified payment, expenditure, effort, or loss:
需要花费：需要特定的费用、支出、努力或损失：
It costs more to live in the city.
在城市居住要付出更多东西
v.tr.（及物动词）
To have as a price.
价钱为：作为价格而具有
To cause to lose, suffer, or sacrifice:
导致失去、受苦或牺牲：
Participating in the strike cost me my job.
参加罢工而使我失去工作
past tense and past participle costed To estimate or determine the cost of:
过去式和过去分词 costed 估价：估计或决定…价格：
The accountants costed out our expenses.
付帐人对我们的花费进行了估价
spare
spare
AHD:[spâr]
D.J.[spe*]
K.K.[spWr]
v.（动词）
spared,spar.ing,spares
v.tr.（及物动词）
To refrain from treating harshly; treat mercifully or leniently.
宽恕：控制住而不严厉的对待；仁慈或宽厚地对待
To refrain from harming or destroying.
免于伤害或遭破坏
To save or relieve from experiencing or doing (something):
使免遭：使解除或免于经历或做（某事）：
spared herself the trouble of going.
她省却了去一趟的麻烦
To hold back from; withhold or avoid:
阻碍…，阻挡或避免：
spared no expense for the celebration.
这次庆祝活动费用不加以限制
To use with restraint:
限制性的使用：
Don't spare the mustard.
芥末尽管使用
To give or grant out of one's resources; afford:
提供：给出或耗尽某人的资源；付出：
Can you spare ten minutes?
你能抽出十分钟吗?
v.intr.（不及物动词）
To be frugal.
节约
To refrain from inflicting harm; be merciful or lenient.
宽恕：免于受伤害；仁慈或宽厚的
adj.（形容词）
spar.er,spar.est
Kept in reserve:
备用的：
a spare part; a spare pair of sneakers.
备用的一部分；一双预备的运动鞋
Being in excess of what is needed; extra.See Synonyms at superfluous
超过所需要的；额外的参见 superfluous
Free for other use; unoccupied:
免于其他用途的；空闲的：
spare time.
空闲时间
Not lavish, abundant, or excessive:
俭省的、简朴的或俭朴的：
a spare diet.See Synonyms at meager
节食参见 meager
Lean and trim.See Synonyms at lean 2
瘦削和纤细的参见 lean2
Not profuse or copious.
不挥霍浪费的或俭朴的
n.（名词）
A replacement, especially a tire, reserved for future need.
备用品：储存以备将来所用的物品，尤指备用轮胎
Sports
【体育运动】
The act of knocking down all ten pins with two successive rolls of a bowling ball.
二击全倒：保龄球运动中连续两滚将十个柱子全部击倒
The score so made.
二击全倒得分

to spare
In addition to what is needed:
有节余：超过所需要的：
We paid our bills and had money to spare.
我们付帐后还有节余