question in tax reversal following, liability, question, company, income

hiphop101

 Q: An analyst gathered the following information about a company:
* Taxable income is $40,000
* Pretax income is $50,000
* Current tax rate is 50%
* Tax Rate when the reversal occurs will be 40%

What is the company's deferred tax liability at the end of year 1?
A. $3,500
B. $4,000
C. $4,500
D. $5,000

这题应该用哪个rate计算呢？什么叫做when the reversal occurs？