The Balance Sheet of Greene, Inc. on January 1, 20X1 showed the following. Assets $30,000, Liabilities $20,000 and Stockholder's Equity $10,000. During 20X1, Greene collected $30,000 in revenue, paid $34,000 in expenses, purchased $15,000 in additional assets on account. The stockholders invested an additional $10,000 cash in the business. What are Greene's total assets at December 31, 20X1?

A. 55,000
B. 51,000
C. 41,000
D. 45,000