F-30 The operating costs related to inventories were NT$24,283 million (including the valuation loss on  inventories of NT$59 million), NT$23,116 million (including valuation loss on inventories of NT$56 million)  and NT$27,046 million (including the valuation loss on inventories of NT$17 million) for the years ended  December 31, 2008, 2009 and 2010, respectively.  a. Land held under development and construction in progress of LED as of December 31, 2009 and 2010  were as follows (in millions):  8. HELD-TO-MATURITY FINANCIAL ASSETS  9. ALLOWANCE FOR DOUBTFUL ACCOUNTS  December 31 2009 2010 NT$ NT$ (In Millions) Corporate bonds, nominal interest rate ranging from 0.76%-4.75% and  1.20%-4.75% for 2009 and 2010, respectively; effective interest rate  ranging from 0.45%-2.95% and 1.00%-2.95%, respectively $  4,532  $  9,868  Bank debentures, nominal interest rate ranging from 1.87%-2.11% and  1.60%-2.11% for 2009 and 2010, respectively; effective interest rate  ranging from 1.14%-2.90% and 1.25%-2.45%, respectively 498  504  5,030  10,372  Less: Current portion 1,100  1,964  $  3,930  $  8,408  Year Ended December 31 2008 2009 2010 NT$ NT$ NT$ (In Millions) Balance, beginning of year $  3,430  $  3,051  $  2,799  Provision for doubtful accounts 501  455  215  Impact on acquisition of subsidiaries 1  — —  Accounts receivable written off (881) (707) (463) Balance, end of year $  3,051  $  2,799  $  2,551  10. OTHER MONETARY ASSETS - CURRENT  December 31 2009 2010 NT$ NT$ (In Millions) Accrued custodial receipts from other carriers $  433  $  387  Other receivables 1,407  1,752  $  1,840  $  2,139  11.  INVENTORIES  December 31 2009  2010 NT$ NT$ (In Millions) Merchandise $  2,034  $  2,147  Work in process 647  765  2,681  2,912  Construction in progress — 376  Land held under development 706  1,237  Land held for development 563  36  Prepayment for construction 99  —  $  4,049  $  4,561 