Which of These Would Normally Be Classified as Direct Materials

Which of these would normally be classified as direct materials?
$\begin{array} { l l l } \begin{array} { l } \text { Sheet metal used } \\\text { in making tractors }\end{array} & \begin{array} { l } \text { Lubricants used on } \\\text { production machinery }\end{array} & \begin{array} { l } \text { Plastic used } \\\text { in making } \\\text { calculators }\end{array}\end{array}$
$$\begin{array} { l l l r } \text { a. } & \text { Yes } & \text { Yes } & \text { Yes } \\\text { b. } & \text { Yes } & \text { No } & \text { Yes } \\\text { c. } & \text { No } & \text { Yes } & \text { No } \\\text { d. } & \text { No } & \text { No } & \text { Yes }\end{array}$$